I was thinking about writing a short story about an interstellar war, but there is something that does not leave me rested:

My invaders (the race of the protagonist) would start the war against a planet of a non-spacefaring but otherwise quite advanced race (let's say late XX. century Earth level) to gain possession of their planet. The point is, they want the planet, as a habitable place, not oil, gold or anything - if they wanted those, they could extract them from uninhabited planets without war. My problem is would it worth it?


Could invaders even win such a war?


  • Interstellar travel is possible but takes a lot of energy to perform. (Like it was done by regular means, only faster.) That means resupply is enormously expensive and almost out of question.
  • Interstellar travel is risky. 2 out of 5 ship does not reach its destination, so they only send large groups of ships.
  • Interstellar travel is fast but not instant. (It takes weeks to arrive to the destination.) The resupply lines are ridiculously long compared to the defenders.
  • No FTL communication (only by the means of couriers - see the previous points on that).
  • The arriving invader fleet consists of three motherships carrying one well equipped army each: 10k soldiers, 100 team transports 100 figther/bomber equivalents and 200 tank equivalents. And every equipment those troops need.
  • Invaders have "SF class" weaponry (ray- and gauss guns, antimatter etc.), but they are forced to use it with care (it they use big WMDs, the goal of obtaining a livable planet can't be reached).
  • Invaders have shielding technology, so regular riffles and grenades are unable to penetrate them but heavy artillery and thermonuclear weapons are still effective.
  • Invaders have a single chance of travelling home. (Either to report victory, to fall back or to ask for reinforcement.) The equipment required for that is expensive to operate (in means of energy) and requires maintenance after usage which is not available at the arrival site.
  • Invaders can enter the atmosphere at any chosen point but they are limited by the rules of physics. They are able to fly ways faster than defenders, but not faster than missiles.
  • Defenders have heavy weaponry, jet fighter planes, nukes and so, and they are frightened enough to use them.
  • Defenders have the population of 4bn and most of them can be recruited to fight.
  • The defending planet has countries but they act in a federated mean.
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    $\begingroup$ @mg30rg - Habitable planets are not rare. New studies have calculated that there are 8.8 billion habitable planets in our galaxy. In addition, any advance race will poses the ability to live in space habitats and/or planets where the conditions are not optimal. $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 16:45
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    $\begingroup$ 40% of ships not making it through is extremely high. No advance civilization with interstellar flight capability will have such high failure rate. Even in the galactic center you would have to travel at light speed if it only takes weeks to get to another star. $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 17:00
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    $\begingroup$ Personally, I'd post questions about what the best way to occupy their planet would be on their version of the internet, and let some sort of mutual-agreement system decide what would be a good way to take over their planet with minimum fuss. Wait a minute... $\endgroup$
    – BrianH
    Commented Jan 7, 2016 at 17:17
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    $\begingroup$ @BrianDHall - brilliant, and incredibly scary at the same time. Imagine the aliens building a memorial dedicated to Stack Exchange: Worldbuilding user XYZ - Without your brilliant ideas this conquest would never have been possible. $\endgroup$
    – AndreiROM
    Commented Jan 7, 2016 at 17:27
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    $\begingroup$ There might be some parallels with the conquistadors of the Americas. No resupply, no effective communication back, help beyond reach, only the supplies you can get from the land to be conquered. I'm not sure the technological factor lines up as well, especially since you're talking nuclear weapons. There's a well known story about Cortez destroying his ships to motivate his men. Might be worth looking into for inspiration. "Most of them can be recruited to fight" for the defenders sounds a bit dubious; a significant portion of the population would be the young and old. $\endgroup$
    – jpmc26
    Commented Jan 7, 2016 at 21:35

20 Answers 20


I will be assuming that problems such as the common cold killing these invaders does not apply.

I actually think you're giving the invaders too many handicaps.

  • 2 out of 5 ships don't make it?!?! That's a ridiculously high probability of death. How in God's name would you convince anyone to board the ships? Who would want to be in the navy?

  • I understand having to use couriers, but with the above likelihood of them actually delivering their messages your communications will be insanely poor.

  • Those mother-ships actually contain very few troops and armaments. Consider that a modern super-carrier has about as much crew as you do soldiers, and carries a LOT more planes. Also, funny enough, the city of New York has 34, 000 uniformed officers. You would need 3 full mother-ships to even bring that many soldiers to Earth.

The Situation

Your invaders have zero chances of winning a conventional war. They will attack, win some major battles, definitely change the planet's political landscape forever, but ultimately they will lose.

Nukes in Space

Humanity can and will use nuclear weapons against these invaders. Some think that the US has already weaponized satellites, even though they've signed treaties to the contrary.

On your own world that might not have happened (weaponized satellites), but believe me when I say that humanity will make it a top priority to get a nuke up to the mother-ships once the hostilities are underway, and you will have a very difficult time stopping them. Remember "Independence Day"? Nothing quite so dramatic would be necessary:

Launch a "smart" missile into orbit from one side of the world where you're not looking (we have nuclear submarines that can launch at a moment's notice from all around the planet). Have it sit in orbit, among all the other junk up there until it drifts closer to your ships, then activate/home in when it gets close enough. Eventually one will inflict massive damage

Nukes on Land

Due to the low number of troops available to them, the invaders will need to concentrate at only a limited number of landing sites. This will make them incredibly vulnerable to a tactical nuclear strike.

Guerrilla Warfare

Look at how expensive in $$$, manpower, and equipment the Iraq/Afghanistan "conflicts" have been to the US military. A homemade bomb buried on the side of the road can take out a main battle tank worth millions of dollars, and its highly trained crew (because, surprise, tanks haven't historically needed much armoring underneath before).

Your force might successfully defeat the main military forces of the invaded world, yet be destroyed one vehicle at a time by the survivors in only a few short years.

Approaching the Planet

Your objective is to take the planet while not contaminating the land mass, or, of course, dying. What you need to do is to absolutely overwhelm the indigenous population before they have a chance to organize, and react to your particular vulnerabilities.

Stealth is Preferable

As you approach the planet try to remain undetected. The later humanity finds out you're coming, the better.

You should be monitoring their communications. Is there talk of strange objects moving through the solar system on the news? Has encrypted traffic through military satellites suddenly spiked? You need to know.


If detected initiate a conversation. Deceive the silly humans with talk of being peaceful galactic explorers in search of new civilizations. You are ambassadors of your gentle and friendly species, bla bla bla.

While military leaders may well be suspicious, the liberal masses - brainwashed by years of propaganda - will demand that the governments extend you a friendly welcome. The military will be crippled in their ability to organize against your arrival (although they will be on alert)

Kill the Earthlings

The only way you're taking the planet is by killing so many humans that their civilization crumbles, and their ability to mount any sort of organized attack is utterly defeated.

Furthermore, the more devastating your initial attack, the less likely you are to have to deal with pesky survivors planting road side bombs, and sniping your troops.

Drop the Hammer

Drop rocks on every major population center in the world. Humanity has no good way of stopping you. Some might have time to evacuate - such as the leaders, to some underground bunkers, but your troops can target those. However, the only way of stopping Billy-Bob and his cousin Bobby-Ray from playing IED tag with you in the foothills of some southern state is to make sure that they never have a chance to get to their pick up trucks and drive up into the hills in the first place.

Even as you are convincing the silly Earthlings of your "good intentions", your ships should be rerouting asteroids from the orbits of other planets onto the capitals and populations centers of Earth. As those are dropping, so should your own fighting forces be bombing remote military bases, targeting carriers at sea, and hunting submarines (if you have a way of finding them).

It's worth noting that dropping asteroids on population centers does more than just the obvious (squishing icky humans). These will cause raging fires which will sweep across the land. You can start massive forest fires which will ravage the countryside unchecked, as response crews are either dead, or busy elsewhere.

Furthermore you will be destroying food production (farms burned by forest fires, etc), as well as transportation arteries (highways and train tracks). With food suddenly becoming scarce, and with no means to transport what they do have, the survivors will face starvation on top of fire fighting, and conducting rescue operations.

Biological Warfare

If you have the means of stealthily approaching the planet then you could potentially end the war in just a few short months, and not even have to fire a shot to do it.

By doing just a little bit of research on humanity's pathetically unshielded internet you will discover that there are some research centers on their world which contain incredibly dangerous viruses and diseases (some have even been weaponized by the stupid bipeds themselves!).

Have a few elite squads storm one of these complexes and gather samples. Humanity won't even know what hit them. By the time they get over their disbelief of an alien invasion they will already be dying by the tens of thousands.

With your ability to spread the virus around the entire planet in only days (have ships rain containers of the virus over every major population center) humanity will have no way of fighting the sudden outbreaks. Some will survive by fleeing to remote areas, but at that point they become easy targets to a few well placed asteroids.

This method is especially effective, as any time guerrilla troops are operating in an area all you need to do is release more of the virus around their general location.

Military Campaign

With humanity crippled (and dying in droves) by either disease, or orbital strikes (preferably both) concentrate your military might in a few areas. Your troops and tanks can move through the desolated areas with impunity, and hunt down survivors using the most brutal methods you can think of. Incidentally, have you heard of air-fuel bombs? If you haven't before, give them a whirl. The things those silly humans can come up with!

Your main threat at this point will be nuclear attacks launched by ballistic submarines hiding in the oceans. With a little care, and the ability to intercept missiles in the high atmosphere, however, you should be fine.


And so, could an interstellar war be remunerative from an economic point of view? Yes - Especially if you take the biological warfare approach.

With humans dying of some exotic disease you can simply walk in and take whatever you want. Their art, if your people are into that. All the gold and other precious/rare metals from their vaults, and factories. The list goes on.

NOTE: Take a look at John Ringo's Live Free or Die novel. In it he deals with an invading alien force (which is pathetic by intergalactic standards) absolutely dominating Earth's military, and basically holding the entire world hostage, while demanding all our precious metals (which we mine for them). How do they do it? By first dropping a rock on some cities, then telling us more will follow unless we comply. Governments around the world will comply.

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    $\begingroup$ Nukes will ruin vast swaths of the planet. Ground invasion is also out of the question for obvious reasons. Biological warfare is the way to go. You can cause major damage without the enemy even knowing you are there. $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 17:07
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    $\begingroup$ @ventsyv - dropping rocks on them, while devastating, does not contaminate the ground. They are a great alternative to nukes. $\endgroup$
    – AndreiROM
    Commented Jan 7, 2016 at 17:08
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    $\begingroup$ @ventsyv - so you know what's going on. What are you gonna do to stop it? Shoot the asteroids out the sky? It depends on how big they are. Evacuate the cities? The rocks will start dropping on wherever you're headed next. Hide in bunkers? There's only room for so many people, and you can target those with your sci-fi weapons instead. With the aliens keeping their distance and simply wearing humanity down with disaster after disaster the governments of the world would surrender before you even inflicted maximum casualties. These disasters also limit humanity's ability to respond to outbreaks. $\endgroup$
    – AndreiROM
    Commented Jan 7, 2016 at 17:21
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    $\begingroup$ @MasonWheeler - Ringo's books all follow the same pattern: the governments of the world are all incompetent, the world is crippled by liberals and social parasites, and only one person (himself, over and over again) holds the key to changing it all. A big disaster facilitates his hero's rise to fame and power. His books have their charm, even though they're pretty ridiculous. On the other hand though, he does make some pretty good points, and describes some things pretty realistically. I think in this regard he more or less nails it, in that most governments would cave to that kind of threat $\endgroup$
    – AndreiROM
    Commented Jan 7, 2016 at 19:26
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    $\begingroup$ According to the sagas, Eric the Red set out to colonize Greenland with a fleet of 25 ships, and only 14 made it. That was a loss of 44%. So it appears people (even humans) will put up with odds like that, under the right conditions. $\endgroup$
    – T.E.D.
    Commented Jan 8, 2016 at 16:25

Unless occupying this planet is vital for the survival of their species, invasion with the aim of occupation would not be worth the effort.

The challenge here is that you’re outnumbered, on foreign soil, at the limits of your ability to resupply, and want to maintain the planet’s status as “livable”. You’re invading their home with the intention of taking it from them — that’s an existential threat that is met by a fight to the bitter end.

Could they defeat the other species? Of course: redirect asteroids of various sizes into collision courses with major population centers. When you control orbit, the planet is absolutely at your mercy. But such a strategy threatens the ability for the planet to be occupied after it’s “cleansed”. If you want to leave the environment intact, you will have to take a much more difficult approach.

Fighting four billion reasonably advanced creatures with an army of 30,000 is folly. Frankly, it’s poor planning on the part of the invaders. Even if you could strategically destroy population centers from orbit, many hundreds of millions will take to guerrilla warfare. Usage of biological weapons has the same problem — you simply won’t kill enough of the population to prevent long-term entrenched resistance.

If you really desire a victory, your best approach would probably be to seek a single continent. With an orbital show of force and the destruction of population centers, you might be able to get the native species to negotiate a peace in which you can occupy some of the planet. Once that has been achieved you’ll have an actual staging area with which to invade the rest of the planet and have the adequate supply lines to dispatch the inevitable guerillas.

  • $\begingroup$ I think it depends on your definition of "Intact". Throwing some asteroids at major population centres will cause desolation on only a couple thousand square km of the earth's surface area. For reference, the earth's surface area is ~ 510 Million square km. Even if you cratered every city and burned all the countryside, most everything would regrow in a few months/years. $\endgroup$
    – Kaz
    Commented Jan 9, 2016 at 13:28

Do the invaders have a compatible biochemistry with the planet? Even assuming they share the same basic chemical structure (carbon based, water-drinking, air-breathers for example) they are unlikely to be effected by native viruses or bacteria (despite what H.G. Wells tells us).

As such they could, with their advanced tech, unleash a modified native virus on the planet, decimating the defenders while leaving the resources and planet untouched.


To clarify, if the engineered virus doesn't kill every defender (and let's face it, it probably won't) it will at least reduce the number of combatants available so a ground war becomes a more viable option. Additionally the infected/dying may further tie up enemy resources as hospitals become overloaded, etc.

If you wanted to be really insidious and the attackers have time on their side, they could abduct members of the defender population before they become aware of the attackers presence in their solar system and introduce a engineered genetic condition that will spread throughout the population over the course of a few years before activating.

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    $\begingroup$ I was going to mention biological warfare too. With their tiny army, sci-fi or not, they're toast. Basic Guerrilla tactics will wear them down long before any kind of victory is in sight. $\endgroup$
    – Cyrus
    Commented Jan 7, 2016 at 16:12
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    $\begingroup$ Tiny army or not, space superiority gives a bit of a tactical advantage. Having to keep the planet intact and livable makes things difficult, otherwise I'd just advise dropping asteroids on the planet from range! $\endgroup$
    – evilscary
    Commented Jan 7, 2016 at 16:15
  • $\begingroup$ I thought basically compatible biochemistry is obvious from their intentions. Also, I assumed their medical science can deal with Earths biological threats. $\endgroup$
    – mg30rg
    Commented Jan 7, 2016 at 16:17
  • $\begingroup$ "they are unlikely to be effected by native viruses or bacteria (despite what H.G. Wells tells us)." - why? If their immune system has no idea how to counter it, why wouldn't they be affected by e.g. decomposition bacteria that can "eat" any organic matter? Viruses, sure, but I'm not so sure which way it would go for bacteria. $\endgroup$
    – Random832
    Commented Jan 7, 2016 at 21:01
  • $\begingroup$ @evilscary I think you overestimate what a few asteroids would do to the planet. An asteroid with the energy of, say, a H-bomb would destroy a city, easily, but within a year, wildlife would've returned/regrown etc. and you'd have no idea it had even happened besides a crater. $\endgroup$
    – Kaz
    Commented Jan 9, 2016 at 13:32


Contrary to the claims of many excellent answers posted so far, it is perfectly possible to conquer a world with the given technology gap. The situation described in the question strongly resembles that of 15th and 16th century explorers and conquistadors in the New World.

Let's compare the conditions to each other:

Interstellar travel is possible but takes a lot of energy to perform. (Like it was done by regular means, only faster.) That means resupply is enormously expensive and almost out of question.
Interstellar travel is risky. 2 out of 5 ship does not reach its destination, so they only send large groups of ships.
Interstellar travel is fast but not instant. (It takes weeks to arrive to the destination.) The resupply lines are ridiculously long compared to the defenders.
No FTL communication (only by the means of couriers - see the previous points on that).

The first journey of Colombus was remarkably similar to these conditions. He spent inordinate amounts of time persuading royal courts to grant him ships to explore the New World, due to the extremely high levels of expense required for the journey.

Out of the 3 ships he brought to the New World, one was wrecked and did not return. Even after ocean navigation was improved more than two centuries later, many ships were still lost while crossing the Ocean Sea. Each journey took weeks, and the ocean journey was the only way information and material could be transferred to and from the New World.

Invaders have "SF class" weaponry (ray- and gauss guns, antimatter etc.), but they are forced to use it with care (it they use big WMDs, the goal of obtaining a livable planet can't be reached).
Invaders have shielding technology, so regular riffles and grenades are unable to penetrate them but heavy artillery and thermonuclear weapons are still effective.
Invaders have a single chance of travelling home. (Either to report victory, to fall back or to ask for reinforcement.) The equipment required for that is expensive to operate (in means of energy) and requires maintenance after usage which is not available at the arrival site.
Invaders can enter the atmosphere at any chosen point but they are limited by the rules of physics. They are able to fly ways faster than defenders, but not faster than missiles.
Defenders have heavy weaponry, jet fighter planes, nukes and so, and they are frightened enough to use them.
Defenders have the population of 4bn and most of them can be recruited to fight.
The defending planet has countries but they act in a federated mean.

Again, this is very similar to the conquests of early Spanish and Portuguese explorers. We can take Cortez's conquest of the Aztec Empire as a case study.

Compared to the Aztecs, the Spanish had vastly superior weaponry. While they lacked true weapons of mass destruction, the difference in firepower between a gun and a macana is not that far removed from the difference in firepower between an asteroid and human nuclear weapons.

The steel armour that the Spanish had also made them highly resistant to the obsidian weaponry of the Aztecs. Similarly, they were still vulnerable to other battle techniques.

Much like the aliens, the Spanish relied on their oceanfaring ships to return home, and if they were lost, they could not be easily rebuilt in the New World. Similarly, the ships allowed them superior mobility compared to the locals, since they could easily sail from one region to another.

Finally, the Aztec governmental structure (a federation of states) is also highly similar to the question.

Therefore, it would be extremely wise for the aliens to take lessons from these ancient conquerors, to not repeat their follies, and to use techniques that have been proven by history.

  1. Use biological weaponry as much as possible.

When your manpower is severely curtailed by supply lines, the best way is to make disease your local ally. The Spanish had smallpox, which greatly devastated the defenders and rendered them unable to effectively fight back. If the aliens are biochemically different from humans, they would need to bioengineer pathogens against humans with their superior technology.

  1. Instill discipline and rein in your greed.

Cortez's famous defeat in La Noche Triste is an excellent example of what greed and angering the locals would do. While the Spanish came to the New World primarily for wealth, the aliens are here to conquer the land. Therefore, needless looting of the locals would only lead to them rising up against you, and superior technology can and will be overwhelmed by numbers.

  1. Make use of les collaborateurs freely, you can always wipe them out later.

Cortez shrewdly allied with the Tlaxcala, a Mexican tribe which opposed and often fought the Aztecs. The alliance was a decisive factor in his survival and later victory over the Aztecs. Making use of human rivalries would greatly accelerate the process of victory in the aliens, for example if there is an ongoing Cold War in the world, the aliens could support the weaker side, and use their help to defeat the stronger power.


3 of 5 ships make it? Sounds like an Islamic-type of theology or outright lies to get your troops on board.

I'm assuming that your invaders don't have biotech or nanotech and can't just plague/sterilize us or build robot factories on the moon to overrun Earth with drones.

Two scenarios depending on the invaders loiter time.

Scenario 1 - long loiter time, coldsleep or food synthesizer tech or something like that:

  1. Drop rocks on population and military centers.
  2. Look for survivors/threats.
  3. Repeat 1,2 until population is attrited down to surrender levels.
  4. Wait for planet to recover from strikes.
  5. Land and party.

Scenario 2 - No Time to waste and have to limit damage to environment:

  1. Rock strikes against hostile military powers/targets with global reach.
  2. Land and establish beachhead.
  3. Conduct conventional operations against C&C and any threats with support from orbit.
  4. Negotiate surrender/truces with those who want to survive.
  5. Destroy the rest.
  • $\begingroup$ Let's go with scenario 2! (The other option is not acceptable for plot reasons not discussed here.) What would stop the inhabitants of the planet to nuke the living sh!t out of our beachhead from a nuclear submarine? $\endgroup$
    – mg30rg
    Commented Jan 8, 2016 at 11:51
  • $\begingroup$ Scenario 2-Step 1: Subs are included in threats with global reach and I assumed can certainly be destroyed from orbit by a spacefaring technology (or a rock for that matter if we don't care about preserving beach houses). $\endgroup$
    – Kevin
    Commented Jan 8, 2016 at 11:57
  • $\begingroup$ How? I mean it is pretty hard to detect a submerged submarine with any regular means. Also nukes are hidden at various locations on the planet and while destroying military bases and big cities is not a big deal finding all hidden silos is nearly impossible. (And you only have to miss one.) I think it is better to stay space based (outside orbit) and only land if necessary. $\endgroup$
    – mg30rg
    Commented Jan 8, 2016 at 12:00
  • $\begingroup$ I agree but the scenario specified little or no loiter time. Ok we can postulate that fleet uses advanced magnetics in their propulsion/shielding systems and have advanced sensors to detect intersteller debris, easily adapted to detect large magnetic anomalies like subs and silos. $\endgroup$
    – Kevin
    Commented Jan 8, 2016 at 12:06
  • $\begingroup$ Also I envision a beachhead in this case as being something like Australia. $\endgroup$
    – Kevin
    Commented Jan 8, 2016 at 12:34

The scenario is essentially about colonialism. As March Ho noted in his answer the spanish conquistadors faced similar odds in the new world against the Aztecs and Incas. What he didn't mention, but really should have, was that in India, Indonesia, and China the technology gap was much smaller no pandemic effect happened and the locals were still utterly dominated. Similarly most of Africa was colonialized despite actually having more diseases than Europe with technology gap somewhere between the rich states of the East and the Americas. Clearly there is huge asymmetry between the the colonial powers and the locals.

And it is basically being able to pick when and where you act against your opponents. The colonial power can attack or withdraw as it wishes. The locals in contrast can act only in response to what the attackers have done. They have no access to the homeland of the colonial power. They can't attack enemy civilian infrastructure other than that built on their lands after it has been taken over. They have no access to the political factions of the enemy and can't "Divide et impera" it.

In contrast the colonial power can freely choose which soft targets to destroy and skip facing soldiers entirely if they wish. They can make alliances with some tribes or rulers and let them do most of the bleeding for them. They can arrange a coup d'etat that gives them control. They can simply bribe key officials. As such, when ever a colonial power appeared the local governments were pretty much doomed to destruction. A strong leader could unite the locals and keep up with the invaders, but the successor of an exceptionally strong leader is almost certainly much weaker and unable to maintain what his predecessor built. So successfully resisting colonialization will only buy you a generation.

I could go further on precise strategy to used by your hypothetical invaders, but there is no point really. The asymmetry of positions is too large, as long as they are patient and don't kill themselves off with pointless warfare they will win. "We come in peace" is a cliche for a reason. It works. Just read any history book about the colonial era for ideas.

Besides your question was more about the financial side. Whether this could be profitable. And it is actually lot more interesting question. Projecting force across vast distances and administering distant lands is actually very expensive. That is the real reason colonialism ended, not people suddenly developing better ethics or natives getting better at resisting.

Which begs the question why did colonialism happen? Simply, because while it is a drain on economy as a whole, it creates huge fortunes to a small section of the population. And people talk about the needs of the many outweighing the needs of a few precisely because in reality the opposite is often true.

There are a few scenarios where this could be true for your aliens:

No effective financial system. On Earth the great fortunes created by colonialism allowed industrialization (and many other things) to be financed from private fortunes. Without an effective stock market and financial system capable of financing private enterprise effectively this was vital and made colonialism overall good business despite by itself being not so good. Your aliens might have a similar situation, if their financial system is retarded in comparison to ours. Religious or ideological reason would be likely. They'd have the technology needed, but choose not to use it.

Strict class divisions. If the decisions are exclusively made by a small ruling class having the project cause a net loss to economy would not be an issue if the cost is spread to entire population while the benefits come to the ruling class. This was actually common for much of warfare before modern democracy. It is fairly simple to make this true for an alien civilization. Many fictional aliens already have elaborate caste or pseudo-feudal systems.

It is gambling. If the ruling class has lots of wealth as is reasonable for an interstellar power, the cost of invasion might not be an issue. Essentially they would be competing with each other for the prestige of having successful colonies. In such a scenario failed expeditions would be written off as part of playing the game. Essentially it would be considered to be the price of being a colonial power, and could be ignored when considering the profitability of successful expeditions. This seems, and is, silly, but it more or less happened in Africa. Which is part of why the colonialism left such deep scars there.


Recent studies, based on newly available data from the Kepler telescope,have calculated that 22% (+/- 8 points) of stars similar to our Sun have habitable planets. That works out to about 8.8 billion planets.[1]

Any advance race will also be able to live in space and/or planets where the conditions are not ideal.

If habitable worlds are not that rare and the logistics/cost of war are prohibitive, it makes little sense to wage war in the typical sense.

The one scenario that's plausible is if you have one way colonization missions that have no other option but to fight. This does not sound plausible if it takes mere weeks to travel between stars; it's just too close. Why not sent a bunch of scouts, explore the system, then report back where the available planets are?

  • $\begingroup$ I can pinpoint a thousand reasons why habitable planets are so rare. Limited range of interstellar travel isn't even the best of them. $\endgroup$
    – mg30rg
    Commented Jan 7, 2016 at 17:03
  • $\begingroup$ If it only takes you weeks to get to another star, you must be in the galactic core where start density is extremely high, thus there will be plenty of habitable planets within reach. $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 17:13
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    $\begingroup$ "Recent studies ...have calculated that 22% of stars similar to our Sun have habitable planets. " - not at all, this is ridiculous. The study estimates that 22% of Sun like stars have Earth sized planets within habitable zones. That's a big, BIG difference. $\endgroup$ Commented Jan 7, 2016 at 18:17
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    $\begingroup$ @ventsyv Well, last time I checked neither Venus nor Mars were habitable :-) Being in a habitable zone is not a sufficient condition for, well, being habitable. $\endgroup$ Commented Jan 7, 2016 at 19:00
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    $\begingroup$ @RadovanGarabík If expansion was a priority for us as a species, Mars would have been inhabited by now. For an interstellar species, settling Mars will be trivial. Not to mention that they would probably also settle Eris and the Moon - lower gravity and lack of air have their benefits. $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 19:10


This simple 4 step strategy enables a relatively minimal force, limited only by your ability to procreate and genetic diversity to eliminate a far larger, but less advanced civilisation. It primarily relies on surprise and technical advantage over numbers.

21st century humans may be able to implement this against 20th century humans with simply with some cubesats, minimal base, population of 1000, several hundred nukes (ideally clean) and a SSD full of knowledge. Increasing population simply decreases risk and time to conquer.

I'm going to focus on these points:

  • Interstellar travel is possible but takes a lot of energy to perform. (Like it was done by regular means, only faster.) That means resupply is enormously expensive and almost out of question.
  • Interstellar travel is risky. 2 out of 5 ship does not reach its destination, so they only send large groups of ships.
  • Interstellar travel is fast but not instant. (It takes weeks to arrive to the destination.) The resupply lines are ridiculously long compared to the defenders.
  • No FTL communication (only by the means of couriers - see the previous points on that)
  • Invaders have "SF class" weaponry (ray- and gauss guns, antimatter etc.), but they are forced to use it with care (it they use big WMDs, the goal of obtaining a livable planet can't be reached).

Okay, so we're very limited in resources and have a large world to attach. Furthermore we are here to inhabit the world, not destroy or mine it. The risks of interstellar travel indicate that for most this is likely a one-way trip - you're doing this to live/die here, it's impractical to jump repeatedly.

Operation Backbreaker

Eliminate the enemy's ability resist you.

  • Collect intelligence about the planet in stealth - even if this means placing sensors a long distance away and getting low res intelligence.=
  • Identify as many major population, energy, trade, intelligence and military centers.
  • Without warning, use clean WMD's against all these targets. (These centers have little environmental value, but huge value to enemy civilisation).

Operation Beachhead

Utilize your technological advantage (huge volume of information can travel in incredibly small volume) to breed and build faster than the native population can recover.

  • Collect intelligence about the planet in stealth (should be far easier now).
  • Pick a location that has natural fortifications, is far from survivors (DO NOT let them organise a resistance against your position) has plenty of resources (including arable land). Australia may be good, with low population, huge resources, and great natural defenses (e.g. ocean, reef, range, desert).
  • Land and build your base.
  • Start building up your resources. In particular... BREED!

Operation Overwatch

Use your space advantage to prevent the enemy from organising.

  • Maintain intelligence collection of the planet.
  • From space, eliminate anything that looks like a threat.

Operation Conquer

Once your forces have built up, increase your holdings and eliminating the enemy once and for all.

  • Using the resources from Beachhead, conquer the planet using conventional military tactics.
  • $\begingroup$ So you've read Footfall, too. $\endgroup$
    – JDługosz
    Commented Jan 10, 2016 at 17:47

You've created quite the painful situation for the attacker. I do believe this species is headed for extinction. Why?

  • They're attacking a planet for habitation, implying that habitable space has value.
  • This is clearly a last ditch effort for a species. Spacetravel is expensive, but they really can only send a handful of soldiers? Obviously this species has run out of resources, and this attack is literally everything they have left. Maybe their planet is under attack itself.

The arriving invader fleet consists of three motherships carrying one well equipped army each: 10k soldiers, 100 team transports 100 figther/bomber equivalents and 200 tank equivalents. And every equipment those troops need.

Multiplying through, our attacking army is 30,000 soldiers, 300 transports, 300 fighter/bombers, and 600 tanks.

Now let's look at what they're up against. Let's say, for sake of argument, only the US gets involved, and compare that against our attackers: (source)

  • 1.4 million front line solders, and 1.1 million reservists, with a "fit for service" population of 120 million. (compare to 0.030 million alien troops)
  • 8,848 tanks (compare to 600 tanks)
  • 41,062 Armored Fighting Vehicles, such as transports (compare to 300)
  • 13,892 planes, of which 2,207 are fighter/interceptor (compare to 300 alien fighter/bombers, which are almost never as efficient as a dedicated fighter or dedicated bomber)

As you can see, if the entire world turned a blind eye to the alien invasion, the US military could lose 10:1 against these alien invaders and still win. And, of course, we're ignoring the fact that the US would not be afraid to use nukes in such a scenario (as you say), so the situation could be even more dire if the aliens try for anything resembling a beachhead.

And the US accounts for about half of the world military, so the alien invaders either need to force a surrender, or continuously win fights with a 20:1 kill ratio. For a comparison, the earliest, most one-sided battles against Al Qaeda by the US were around a 100:1 ratio. So yes, these kill ratios can be done, but it was done by an entire well equipped military raining hell on a rather backwards group of individuals. Its quite clear this group of aliens will not fight the world military for long, they need to do something to disrupt humanity, so that the military falls apart.

Now let's go back to why we know this is a species headed for extinction:

Invaders have "SF class" weaponry (ray- and gauss guns, antimatter etc.), but they are forced to use it with care (it they use big WMDs, the goal of obtaining a livable planet can't be reached).

The way this is worded leaves questions. Obviously "SF class" is a wide wide wide category of weapons. However, do they have any mid-sized weapons? And by midsized, I mean "can take out the entire city of Tokyo or Washington DC." Maybe something that can take out a state. If they have such weapons, the fight gets ugly for us fast. All they have to do is land fast enough that we can't set up an offensive, then go guerrilla. We worry about terrorist groups getting a hold of nukes? How about an alien, who clearly is willing to die for a cause (40% attrition rate in travel), able to strap a state-killer to their back and take out New York. No, not the city. The state. That'd create enough havoc to allow them a certain level of freedom of mobility.

If they lack such capacity, it once against shows how desperate these guys are. They send 3 underequiped ships without the proper sized weapons to subdue a planet? Clearly they just ran out of resources.

So how could they win?

Step 1: biowarfare. Skip the tanks. Skip the weapons. If you're advanced enough to FTL, and insane enough to be willing to roll the dice every time you do, you've got the technology to engineer a brutal virus. Make something nasty, that could win at Pandemic, something so subtle that not even Madacascar closes their ports. Something with a really long gestation time before turning into something that makes Ebola look like the common cold. Make sure the military is not operating at peak capacity when your soldiers actually hit the battlefield.

Step 2: Shock and Awe. Grab your biggest non-planet-killers and go mess up some states. You don't need the whole planet hospitable right? Go wipe Japan off the map. Sink it into the ocean. Make sure Washington DC, New York, LA, Moscow, Bejing, and London populations are turned into crispy critters.

Step 3: Remove the US navy. You didn't need all those troop carriers did you? Turn some of them into suicide weapons to hit the major nuclear carriers, such as those of the United States. Carriers are pesky floating cities at sea full of people who have been trained for years in the best techniques for making your life miserable. Better to be done with them.

Step 4: Landing. Spread your troops out. You're basically going to spend most of your days killing people, until morale gives out. You said they have good shielding, so you don't have to worry about the people killing you, only the military (and the few gun nuts who happen to have artillery...). Assuming all 30,000 of your troops survive, you're going to have to kill people at a rate of at least 1 every 2 hours, every hour, every day, just to counteract humanity's ability to reproduce. You'll probably focus on cities, to make sure your rate is acceptable. At this point, a nice alien army would sue for peace, but its clear your aliens are beyond logic, so I'm assuming they wont stop until all of humanity is dead. If they were smarter than that, they'd have brought a better military.

Step 5: Wait (aka step 1b). Remember your virus? Guess what, it's probably done almost all of the work for you. Your boots on the ground (and perchance for removing states from the map) are going make it hard for the inhabitants to make a good quarantine effort.

Step 6: Do something? What was the alien race's long term plan? They attacked a planet that had to be habitable by the end, using a vastly under-equiped army that forced them to basically go door-to-door scorched earth to do their job... and now what? If you had enough resources to do something with the planet, you probably would have sent more soldiers.

Seriously, what is this alien race doing? Is this just a game, like a bunch of military jerks playing chicken with FTL to go beat up on some planet just to show off the size of their ray gun? The battle may be part of the story, but the rest of the story really needs to explain why this insanely unusual loadout was the army-of-choice for the aliens.


The traditional way for spacefarers to beat a planet is to drop rocks on it. But in this scenario, you specifically want to kill all the inhabitants but leave the planet inhabitable. Dropping enough rocks to kill everyone would probably not leave the planet inhabitable.

You've specified that the invading fleet has some military force, but not nearly as much as the planet does. But I'm confused that the invaders are using military at all. In the future, wars will be fought with drones.

We have combat drones even now, with year-2015 technology. We're piloting them manually (from afar), but that's not a technological limitation; we could build fully autonomous killer drones if we wanted. Your space age invaders, with "SF class weaponry", will be fielding a force of killer robots.

The great thing about killer robots (from a certain perspective) is there's no limit on how many you can make. The first thing the aliens do is they get their killer-robot factories set up -- in an asteroid belt, on a nearby planet, on the moon, or even just on a deserted island that nobody's watching. The aliens spend as long as they feel like, making as many drones as they feel are necessary. Then they push a button and send all the drones out at once. It's a pretty horrible thing to do, but it's very effective.

I expect the aliens' main concern is to prevent the planet from launching its nukes. The planet might launch its nukes out of sheer panic, or they might deliberately render their planet uninhabitable out of spite. The aliens park their ships in orbit around the planet, and if they see a missile go up, they try to shoot the missile down. This might or might not work, but they have to try it if they want to take the planet intact.

  • $\begingroup$ Drones can be fine, thanks for the idea. To be honest, I was thinking about tiny task forces of genetically enhanced massive alien troopers wearing power-armor and wielding gauss-guns, antimatter grenades and personal kinetic shield generators, with massive air support of fast moving fighter/bombers. My invaders would not be outgunned if they are outnubered 1:100 and were able to perform a strike at nearly any point of the planet. (see also: Death from above. :D) $\endgroup$
    – mg30rg
    Commented Jan 8, 2016 at 8:42
  • $\begingroup$ I'm not sure if "shooting missiles down" really helps if the nukes are targeting the planet itself. $\endgroup$ Commented Jan 9, 2016 at 10:46
  • 1
    $\begingroup$ It totally does help! Nuclear missiles have a complex detonation sequence which is necessary to cause fission to happen. Blowing up the missile prevents it from executing its detonation sequence properly, so you can get less fission or none at all. $\endgroup$
    – Dan B
    Commented Jan 9, 2016 at 18:44

I disagree with parts of the accepted answer, so lets add an answer of my own.

No space nukes. Humanity can not strike any moving target in space. Sure, they can place a nuke at a specific point in space at a specific point in time, if they know their target will be there at that time, like a comet. But this ignores that evading is ridiculously easy in space. Space is big. Really big. Just nudge the spaceship a tiny bit in one direction, and five minutes later you're in a completely different place, especially if we're talking distances where nukes matter. Speaking of which, the distance at which a nuke matters is much smaller in space than on earth, because there's no pressure wave.

Guerilla Warfare is not effective. The main problem with guerilla warfare among humans is that combatants hide amongst civilians who you cannot harm. In stark contrast, the very purpose of the alien invasion is to harm the civilian population. It should be obvious how this makes guerilla warfare significantly less effective. As an alien if I can chose fighting a 10'000 strong army or 1000 groups of 10 guerillas who I can easily track with my alien tracking tech, guess which I prefer? Oh, keep in mind the aliens are invulnerable to anything with less of a punch than heavy artillery.

Sadly the question is unclear about how much we can damage the ecosystem. Instead of throwing rocks at cities, I'd just throw about 10-20 rocks into the oceans. It's cheaper and creates far less problems with nuclear winter and destroys cities, warships, islands, and much more.

And no, humanity does not have any feasible defense against asteroids. If done right they wouldn't even spot them before it's far too late.

Wait 15 years, and repeat. At that point civilization should have crumbled due to all ships and shipyards being destroyed which kills most of the existing trade, ownership of land falling apart, because most people owning anything off the coast are dead now, and finally money disappearing because of hyperinflation in each and every currency other than physical silver and gold, since all banks go bankrupt after losing too many assets and securities for loans. The highest risk you face is that they might have used nuclear weapons on each other, which makes the planet less habitable.

At that point you use no more than 5% of your force to ally with some of the local kingdoms and help them to destroy the remaining military of everyone else. Once you're done you have all the intelligence you need to wipe out your allies with the 95% of your forces they didn't know about.


Here's probably the most effective way that you could conquer that planet, with those limited forces, and have a good story:

Step One-Show up(And hope to hell your motherships aren't part of that 40% that doesn't make it).

Step Two-Locate enemy supply depots, communication hubs, power grids, and transportation networks. Destroy these. The enemy will have no clue what's happening, and thus won't be able to respond.

Step Three-Begin an air superiority campaign. Even though you have very few aircraft, they probably will be so much more advanced, that it won't matter that you're horribly outnumbered. Then take out enemy missile launch sites, nuke stockpiles, aircraft(while they're still in the hangers), and any enemy military vehicles, fortifications, or large troop concentrations. Your fleet in orbit can help by launching orbital strikes as well, regardless of what specific weapons you decide to use.

Step Four-Now, find the biggest breadbaskets on the planet, and loot them. They won't be able to resist you by this point. Now, give that food to all of the starving people of the planet, and promise them better lives in exchange for fighting for you. Since you're the one giving them free stuff, they'll do what you say, in hopes that their children won't have to starve. Also, collect anyone who isn't fit to fight(elderly, cripples, children, etc) and secure them in hidden, fortified positions.

Step Five-Now that you've turned the enemy's numerical advantage against them, cross out any remaining loose ends, and secure the planet. Now, if those masses refuse to follow you any longer, or if they realize that they just helped some maniacal invader take over their planet, and decide they're in the mood for guerilla warfare, remember step four. You have their families at your mercy. Use them as hostages. No one but the most deranged fanatic will sacrifice their own children to defeat you.

Step Six-Use the humans as slaves. Work them to death. You won't have the resources in your minuscule fleet to just start making robots to do the job, so you'll have to have someone mine those for you initially. Now, you control the planet, have the humans under control(for the remaining period that you'll need to control them, as they'll be dead from you working them like that soon enough, and they most likely won't have many more children. If them procreating is a problem though, just chop off their balls. Simple, and no research into a sterilizing agent is required.)

Step Seven-Now that the humans have mined the resources you need, use nerve gas or something on them. Or just take them away, group by group, and shoot them, burying them in mass graves. You can take the hostages and stick them in your species' equivalent of zoos or something. Use your newly acquired robots to build an industrial base on the planet, and complete your colonization.

And this, is my quick, seven step program for conquering primitive planets, with miniscule forces. Follow it, and soon you'll be subjugating worlds in no time!


TL;DR: Read "Footfall" by Larry Niven and Jerry Pournelle for 93 cents:


Longer: While destruction of highways, bridges, power generation, centralized broadcasting facilities and key junctions of railroads etc. would destroy our current economy (as in the book), which is highly vertically-integrated, there would be a "twilight period" where still some great resistance could be mounted.

As economies switch from semi-global (average American carrot travels 1800 miles) to local, reliance on products propped up with cheap energy would lessen. There would be a couple really hard winters and a reduction in population, but then the remainder (IMHO) would be able to harness what was left of the "carcass" or our current civilization and "spend down" the equity of working new and used parts, for example.

The conquerors would be busy with tenacious little blobs of people, while some would have the opportunity to learn more about the enemy and find a way to make a meaningful resistance.

If the enemy is so advanced (e.g. Childhood's End Overlords) then, well, I give up. They are nearly godlike....

...but could they monitor/correct everyone being "bad" all at once? Probably not, but, they could just split the planet in half.

So, to answer the question, war, as an economic boon, is always at least a three party transaction. Conqueror invades victim, extracts something, and uses that resource to do pay a third party to do X. Poor person plays lottery, buys insane yacht, guys at yacht factory get work, their families eat, and maybe, buy lottery tickets.


  1. Destroy the 2nd temple of David and build the Flavian Ampitheatre (aka "Coliseum")
  2. Invade France to grab 72% of the world's [then] government-owned gold
  3. Invade Iraq, and pawn off some crap 2nd-gen cellular equipment, as part of many examples of carpetbaggery
  4. Conquer Jerusalem and cart off the Hebrews to (largely) muck out the topsoil-filled drainages of Babylon and tending to date palms through moderation of their fertility (to prevent over-fruiting) and poor quality

If you invade someone, and take all of commodity X, then the market crashes. If you kill off all the customers, the market crashes. If you make movement impossible, there is no market.

  • 4
    $\begingroup$ This is more of a comment than an answer. Consider fleshing it out. $\endgroup$
    – AndreiROM
    Commented Jan 7, 2016 at 22:43

If trade and mining isn't realistic (as have been previously discussed here), war certainly would not be. You might consider war a special case of trade.

It's worth reviewing the answers to these previous two threads for general issues on the logistics.

  • $\begingroup$ All those answer ate related to no FTL travel. In my construct FTL exists. $\endgroup$
    – mg30rg
    Commented Jan 7, 2016 at 17:26
  • 1
    $\begingroup$ @mg30rg - I think the "No FTL communication" tripped up a lot of people.When I saw that I assumed that to mean no FTL travel; I don't see how you can have one and not the other (couriers do count as FTL communication by the way). $\endgroup$
    – ventsyv
    Commented Jan 7, 2016 at 17:42
  • 1
    $\begingroup$ @ventsyv The criteria was exactly "No FTL communication (only by the means of couriers - see the previous points on that)." we also no there is only one chance for them to travel back, and they hardly will use that to send an email. $\endgroup$
    – mg30rg
    Commented Jan 7, 2016 at 17:48
  • $\begingroup$ Hmm, if the travel cost is not "realistic", you can make it as low as needed to fit your story. So the question isn't "is it practical" but "what precisely should the FTL travel costs/logistics be to make my story happen." $\endgroup$
    – JDługosz
    Commented Jan 8, 2016 at 18:19


Sprinkle the world with Neutron bombs and civilization and most people are gone while leaving the planet inhabitable. The resulting fallout will be far less then traditional nuclear weapons and any sufficient advanced civilization will be able to deal with it.

  • $\begingroup$ Please elaborate a little more on this answer and provide some links too. For instance, (I suspect) even dropping a substantial number of neutron bombs is unlikely to kill more than 1/2 the human population. That would leave the other half pretty pissed and motivated to fight, I think. What is the "death zone" of a neutron bomb? How long does the neutron activation leave the location uninhabitable? $\endgroup$
    – Jim2B
    Commented Jan 9, 2016 at 13:41

Why invade when you can trade?

Consider the Roman and British empires. In many ways different, not least because of different technology levels. But what they had in common, was that they colonised with the help of existing local rulers. The offer was basically "subjugate yourselves to our empire, and get the benefits of being part of it. We will maintain the peace. You will pay fair taxes for this service. We will both benefit by trade. You will remain rulers of your country and we will not interfere if you stick to the terms of the agreement we will now negotiate. Refuse, and we'll offer these terms to your neighbours and help them conquer you. Accept and later rebel, and you will be wiped off the face of the planet. "

Getting started from just a few ships is the hard bit, especially if there is instinctive xenophobia to contend with. But you can probably find a powerful or power-seeking person somewhere who you can work with in secret. Especially if you have stealthy technologies to conceal a few comings and goings, and more advanced communications and computers so you can subvert their Internet.

(For all I know this is happening here and now. UFOs, social media, rich people who act as if they are used to living on a different planet... but we are quite capable of all that crap without invoking lizard-people)

Further down the road when you have successfully joined this planet to your interstellar empire, there are two variants. Good faith, where the Empire is as it appears. Bad faith, where the invader's real plan is xenocide. This requires that all the invaders keep the secret until they are ready to strike....

  • $\begingroup$ "Why invade when you can trade?" Because you don't want those pesky, unhygenic locals in your shiny real-estate? $\endgroup$
    – mg30rg
    Commented Feb 7, 2017 at 10:47

Like other posters have said, militarily the invading aliens don't have a chance unless they get rid of the rules their politicians must have put on them. But, they can still win:

  1. Threaten and bluff. Get a 1 ton chunks of moon rock and start dropping it on high value targets. Get the Earthlings to surrender without a fight. Make an antimatter bomb demonstration strike in the middle of the ocean to make sure they get the point. Then pull the victim card and say you're the last of your race, 'sniff' and all you want is to live in peace and harmony.

  2. If that doesn't work, mine an asteroid for more platinum than 800% of world output per year. Then buy a country. Make it a living paradise with your advanced energy technology and unbelievably rich with your space mining capability.

  3. If that doesn't work, look right, look left, and make a dope deal with an Earth government to sell a mothership. Make yourselves unbelievably rich with asteroid precious metal mining and advanced technology sales. All 30,000 of your troops will retire in luxury. Offer a huge but sparsely populated country like Canada, Australia or Russia the opportunity to host more of your incredibly rich countrymen (settlers from the homeworld, shhh) in return for being the only Earth vendor of exclusive alien technology. Write up some idiotic cockamamie report for the high command about how the loss of the mothership somehow isn't your fault, then enjoy your new life of luxury on Earth.


If you want to take down a building don't take a sledge hammer to it. Collapse its pillars, watch it fall, then take the hammer to the remaining rubble.


Conventional invasion is all but impossible due to the severely undermanned and underteched invaders. Any large usages of WMD will cause major ecological damage that may not be recoverable or economical. With this in mind, they are less of a conventional invasion force and more of a task force.

1. Sow chaos and distrust

I would just instigate a bunch of wars or WW3 on the planet. Wait until things heat up and then make a solitary heavy strike on a major capital. Make sure the strike is such that noone knows where it comes from, but especially not from potential aliens. Bonus points if you can pin it on one of the major nations. This should cause wide spread confusion, chaos, distrust, and kick the wars into high gear.

2. Break the back bone

Once everyone distrusts one another, is at odds, and lacks clear information, begin widespread orbital strikes on major communication, political, transportation, and military centers. These strikes would be the size of tactical nukes (think Rods From God), causing massive damage but not enough to screw with the surrounding area.

3. What Follows

You have now destroyed or neutered every major government, and they all hate or distrust each other. The infrastructure is crippled, and people are going to quickly start starving. As people start dying, chaos will break out and the governments will have severe issues maintaining any order at all. People are not going to rally against the alien invaders because:

  • Countries do not trust each other
  • Communication is severed so organization will be difficult
  • Governments are all but gone
  • There is nothing they can actually do
  • People likely have no idea what is going on
  • Starvation starts almost immediately.
    • If you prevent supply chains from forming, there can be no large scale populations. They either die or disperse searching for food.

4. Shooting Rats

Afterwards, limit yourself to occasional strikes to prevent attempts at linking communications, creating supply lines, or just organizing an effective government. Sit back and wait for most of the population to either die of starvation or kill each other over the scraps. At this point, move in and sweep major areas clean of people. This process will take years, and this is not a war. This is an extermination. Guerrilla warfare will not be the issue people think it is. Guerrilla warfare mainly matters when you actually care about collateral damage. If you really don't care, and in fact want to kill everyone it is much much easier to win that war.

Picking Daisies

By the time you put troops on the ground, there is no army. There are only remnants using salvaged weaponry, which will mainly be light weaponry. Joe Smoe is not going to be flying a jet or driving a tank, let alone being able to arm or maintain them. Not everyone will fight, many will run or just collapse in fear. Potential fighters != fighters != a soldier.


For simplicity, I'm assuming your aliens have targeted the Earth.

Your aliens need to avoid making the mistake of making this a "war to the knife". That is, if they are trying to destroy the entire human race, the humans will, predictably, resist strenuously. They'll fight with overwhelming numbers, they'll learn from your technology, and they'll haunt "cleared" areas assassinating you. Your small colony force won't win a battle of attrition.

I'll recommend re-examining your motives before deciding on an approach.

Is the extirpation of humanity really necessary?

This is the key question. If your heart really can't handle sharing a planet, well, okay. You don't want to fight a conventional war, for reasons given above. You want to break human civilization without letting them get any licks in. So...

Invest heavily in orbital bombardment. Every city, dam, bridge, railroad junction, and power relay station is targeted. Basically, do as much damage as you can to industrial and trade infrastructure. Then bombard the planet indiscriminately with meteorites, and laser the ocean. You want gigantic clouds of steam and dust to make agriculture impossible for years.

This is basically a synthetic dinosaur-killer asteroid approach.

This will destroy most of the humans. There will be some left, even some pretty tough enclaves here and there using hydroponic farming, but you'll have wiped out most of them, and the remnants will definitely be on the back foot. It'll be more of a mop-up operation than anything else.

Now the ecosystem will be a little bit trashed, but what's that to you? You have no sentimental attachment to any given species; whatever remains will suffice. Expect rapid speciation of surviving animals.

Are you just looking for a place to live?

Your colony numbers tens of thousands only. How much room do you actually need? Might be you could get some benefit from technological or cultural interchange. So...

Figure out where your colony needs to be, depending on your physiology. Are you cats tropical guys? Or do you like it cold -- you might get lucky, and annex Antarctica. Nobody is going to fight too hard for that.

So pick a place. An island is best, easier to defend. Let's say Britain is the place where you feel you would be happiest. State that you are going to expel the population from that area in phases...

  • Phase 1 -- With your space resources, you will subsidize people to peacefully leave your new homeland. You might be surprised how many people will take up your offer. Any hey, Europe is all about taking in refugees, right? It would be churlish not to accept Englishmen. This phase will last for 1 year.

  • Phase 2 -- The island will be embargoed. All ships or planes moving in or out will be destroyed. This phase will last for 1 year.

  • Phase 3 -- Anyone remaining will be destroyed. All buildings will be destroyed.

  • Phase 4 -- Land your colony and start engaging in profitable trade and student-exchange relations with the rest of the planet.



Not by invading us.

If they wanted to kill us, they would just need to send some asteroid at relativistic speed towards the earth. Accelerate an asteroid of a few km radius to 90% the speed of light and point it towards the earth. The impact will be 10x worse than the one that killed dinosaurs.


It would be like the Romans considering invading North America when they can barely survive and navigate the high seas and when their is plenty of land and enemies to conquer on their own continent. It s just too early.

Given all the handicaps you mentioned, it would be much smarter for your interstellar Civ to focus on maximizing their own star systems and the neighboring ones.

Given that just in our system their is enough resources to build enough O'Neil cylinders habitats to house a million time the current population, if your interstellar Civ is rational and lives in a similar system, it would first focus on maximizing it s current system. The steps would be

-build a Dyson swarm of habitats around the Sun(s)

-upgrade it to a Dyson sphere

-do the same for the nearest star

-rinse and repeat

By the time they Dyson sphered a few stars, they would probably reach type 3

By that time, humans and earth would be quite irrelevant to them. They could easily kill us or not. To them we would just be like some ant colony.

  • $\begingroup$ Yes, so my invaders are more "aggressive real estate agents" than invaders in the classic manner. $\endgroup$
    – mg30rg
    Commented May 18, 2020 at 9:30

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