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In my setting, a near-human species attempts to invade modern-day Earth. With a few, very strange, outlier exceptions they are limited to WWII-era technology, and cultural/societal restrictions keep them from adapting and improving beyond this. The exceptions are the absolute minimum necessities that allow for large-scale space travel, and planetary take off/landing. These processes are generally crude in nature, but effective enough to get the job done.

They have the ability to land their WWII era aquatic ships in the oceans in a controlled descent. They have the ability to send in aircraft in drop pods that deploy once reaching a proper altitude. Similar technology allows paratroopers to be deployed. Other land forces must be deployed from a water ship. Communication with the orbital fleet is limited to communication aquatic ships and communication towers that can be established planetside. Radio and encryption technology are otherwise roughly on par with WWII.

They have WWII-style artillery that can fire from space. Similar to America during WWII, they have used computers to calculate required trajectories for angle, elevation, etc, and fire the shells according to lookup tables and books. The largest space artillery pieces equaling that of the Schwerer Gustav. The shells are specifically designed to withstand atmospheric entry.

Edit: Per discussion with Mołot, the accuracy could be no more than a 20 mile radius. The time from firing to impact would be roughly half a day, allowing developed nations plenty of time to respond. Anti-ballistic missiles would be a natural defense against the artillery, against which the only real counter (that I can think of) is to simply exhaust the anti-ballistic supply with enough artillery shells.

On principle of honor, they refuse to perform acts of mass destruction or non-traditional warfare, such as gas attacks, atomic warfare, or accelerating large objects into the surface of the Earth.

Potentially I can make the invading force number millions, billions, or trillions in size. Similarly, the ammount of non-perishable supplies they bring with them can upscale as needed.

The goal of the invading force is to subdue the armies of Earth and occupy the planet, not mass genocide.

Ignoring the logistical issues with commanding and mobilizing forces on such massive scales with such antiquated technology, how much larger must the invading force be to be more or less on equal footing against Earth? What would be the primary challenges this invading force would face?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Serban Tanasa Oct 4 '16 at 15:06
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    $\begingroup$ Due to the technology gap, your answer is actually going to be much the same as "how big would a WWII era-army have to be to conquer the modern world?". To which the answer is arbitrarily large. The technology gap is just too big. A single modern apache helicopter could take out an entire WWII era tank battalion. A single modern aircraft carrier could destroy an entire WWII era navy before it can even get in range. A single F-22 Raptor could take out an entire WWII era air wing. You get the idea. The only advantage the aliens had was orbital bombardment, but you already ruled that out. $\endgroup$ – Kaz Oct 5 '16 at 12:23
  • $\begingroup$ @Kaz A single F-22 Raptor doesn't have enough ammunition to take out an entire WWII era air wing. Don't ignore the logistics on the Earth-side, they are crucial here. $\endgroup$ – Luaan Oct 6 '16 at 10:18
  • $\begingroup$ @Luaan Fair point. 8 missiles, 5 seconds of continuous cannon fire. probably downs only 20 planes per sortie. $\endgroup$ – Kaz Oct 6 '16 at 10:38
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    $\begingroup$ @Luaan IMO, it still comes down to arbitrarily large, where you have so many planes/tanks/people etc. that you can literally reinforce faster than the enemy can shoot bullets. Especially assuming you can't land right on top of an installation and/or it's got dedicated AA batteries (or has a standing force, or is anywhere near a fleet, or is within range of other bases/planes/drones/whatever. Not to mention what some cruise missiles could do to a large mass of enemies). $\endgroup$ – Kaz Oct 6 '16 at 11:41
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Invasion from space obviates difficulties with covering ground and intelligence, so that's nice. The difficulty is really how big your army has to be to totally and completely subdue all human societies capable of resisting in a serious manner. There are 7.5 billion of us here; let's say of that, about 4 billion of us will be seriously capable of resisting in some capacity or another. There were about ~550 million people in Europe at the start of WWII; it took a German army of about 13 million, plus the armies of Italy, Romania, etc. to subdue that continent almost completely; I would estimate 20 million a good count.

If it takes an army 20 million strong to subdue 550 million humans, then it seems like it ought to take 145.5 million troops with accompanying materiel to bring Earth's fighting population to its knees and keep it under control.

However, Earth will use nukes. It will use chemical warfare. It will carpet bomb your troops stationed in human cities. It will steal your technologies and use them against you in novel ways. It will employ fanatical, brainwashed children with bombs on their chests in your alien management centers. Earth will dig itself underground. It will scorch its own infrastructure and commit every kind of sabotage and betrayal imaginable. Its religions will make of you devils, its languages words of hatred reserved for you, and its nations will define themselves by the bodycount they reaped before they fell. It will stop at nothing to oust you from its only planet.

Triple the number.

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    $\begingroup$ While I love your answer, don't you think some human may want to work with the aliens? It may not be popular, but WWII saw collaborator in country Germany invaded, that helped them tremendously in keeping the populations in check. $\endgroup$ – DrakaSAN Oct 4 '16 at 8:43
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    $\begingroup$ @KillianDS it is very hard to determine the tech level disparity. On the one hand, they have certain technologies we can only dream of. On the other hand, they seem artificially hamstrung in the warfighting department. I will say this - if they are tactically not brain-dead, they will focus overwhelming force on us piecemeal, defeating us in detail rather than all at once. In that case, if an army larger than the total population of the US came crashing down anywhere on Earth, resistance would be futile. We simply couldn't kill them fast enough. $\endgroup$ – Adam Wykes Oct 4 '16 at 14:19
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    $\begingroup$ @enderland yeah there are big problems with imagining a scenario in which, SOMEHOW, a Nazi army the size of North America's total population basically materializes out of thin air anywhere on Earth and yet doesn't retain any real technological advantage... I would not read a story based on OP's premise. $\endgroup$ – Adam Wykes Oct 4 '16 at 17:19
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    $\begingroup$ Romania was definitely on the Axis side for most of the war. A quick google search or wikipedia hunt would have given you this information. As for the exact size of the German army, this is actually a more difficult number to ascertain accurately, but I just used the first estimate I was provided with in my quick google search on the topic. Someone with better knowledge is welcome to step forward. Those who would rather simply like to guess I am wrong without even the basis of a google search are welcome to stay quiet. $\endgroup$ – Adam Wykes Oct 5 '16 at 9:01
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    $\begingroup$ I loved your last two paragraphs, wonderfully well written - both oddly pride inducing and also terrifyingly shameful at the same time.You're absolutely right though - many (maybe even most) modern humans would rather die fighting than live as slaves and history has shown time and time again that a good rebel force can ruin a modern military presence. $\endgroup$ – Dan Oct 5 '16 at 11:17
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No chance. As soon as they are in orbit, a few US/Russian/Chinese/British/French/? missiles with nuclear warheads induce a Kessler syndrome, and that will be the end of it. https://en.wikipedia.org/wiki/Space_debris

To make this a bit more clear:

Debris orbiting earth occasionally hits other debris, thereby increasing the number of particles. Over time, the smaller particles are decelerated in the upper atmosphere, and burn up as meteors, or are blown away by the solar wind. If the number of debris particles gets too high, however, there is a runaway effect.

The original effect predicted by Kessler would not be nearly as bad as to pose a real threat, but here the space junk is not aluminium and plastic from old telecom satellites, but megatons of steel from that invasion army & navy.

Those WWII spacedonks are either very lightly armoured, and get killed by the first hit on their ship, or heavyily armoured, and maneuver very slowly, and get hit so often they still die. Every ship has a lot of fuel, explodes, and increases the junk density ever more.

I guess you wouldn't even have to shoot at them. In thousands (millions?!?) of ships, a few inevitable accidents with following explosion would start Kesslers cascade.

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    $\begingroup$ Nope. One the cascade is started every other ship that comes close is shredded into small pieces, and makes earth even more inaccessable. They can send in as many troops as they want, until every earth man is killed by falling space junk (i thought they dont want that?), they will never come through. $\endgroup$ – Karl Oct 3 '16 at 22:13
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    $\begingroup$ @BryanMcClure Using shields made out of a handwavium/unobtainium alloy. $\endgroup$ – Michael Hampton Oct 4 '16 at 3:39
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    $\begingroup$ @NexTerren The more you bring the worse the Kessler syndrome gets and the faster they die. Sacrificing ships makes it worse, in effect every ship they feed into the maw is turned into a weapon guarding Earth. $\endgroup$ – Loren Pechtel Oct 4 '16 at 4:36
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    $\begingroup$ You misunderstood the implications of Kessler syndrom. It means it becomes impractical to stay in orbit for extended periods of time. Simply crossing the affected zone would be dangerous, but not impossible at all. $\endgroup$ – Antzi Oct 4 '16 at 5:18
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    $\begingroup$ @Karl From the wikipedia article "However, even a catastrophic Kessler scenario at LEO would pose minimal risk for launches continuing past LEO, or satellites traveling at medium Earth orbit (MEO) or GEO. The catastrophic scenarios predict an increase in the number of collisions per year, as opposed to a physically impassable barrier to space exploration which occurs in higher orbits." $\endgroup$ – Taemyr Oct 4 '16 at 10:29
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The aliens are toast, in so many ways.

Air

Modern air combat is all about missiles, and WWII era planes have no defence against them. One shot, one kill. Even at guns range it's vanishingly unlikely that the aliens would get a single kill against a jet. Possibly against a helicopter, but even then it's questionable.

And that's before we look at all the various ground-based options. In WWII there were planes shot down by infantrymen with rifles, never mind modern anti-aircraft guns or SAM batteries.

Basically, humans own the sky. Which means...

Land

... anything which moves on land is dead. WWII proved that you need at least a neutral sky to have a chance on land. If the enemy owns the sky, it's game over. Cluster bombs, napalm, phosphorus - alien infantry will be annihilated. And WWII era armour doesn't stand a chance against an Apache. Humans also have the option of tactical nukes, but the aliens would be dead long before that.

Modern infantry weapons aren't that much better than they had in WWII. But of course it's not just infantry versus infantry. Modern land combat is about manoeverability. The aliens simply wouldn't be able to land a shell on a tank from orbit, whereas air superiority means we can pick them off at leisure.

Of course getting onto land means you have to land on the water first and get ashore. But...

Sea

WWII proved decisively that the only way to win on the water is to either be a carrier group or a submarine. We own the air, so that option is out. Us Brits got ASW up to a standard by the end of the war which made WWII-era subs just a complicated form of suicide, and modern ASW is a whole lot better even than that. If it's in the sea or on the sea, it's dead.

Literally, the only way in which the aliens could beat humans is for there to be more aliens than the world has bullets.

So, having established that they can't land and beat us...

Space

We have plenty of means for getting into orbit. Currently it's all geared around avoiding losses, but if you're in a wartime situation then that doesn't really count. If the aliens can release drop-ships, then there's a way back inside too. A few Space-X Falcons filled with SAS/SEALs/Spetznas will make short work of them.

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  • $\begingroup$ Regarding air, the main opportunity for the aliens would be bombing ground bases. If you send in enough planes you'll do some damage, eventually, assuming that no one perfects laser defenses. Eventually you'll run out of AA. $\endgroup$ – enderland Oct 4 '16 at 17:02
  • $\begingroup$ @enderland When there is any chance that your army does not have 100% air control, all barracks, airbases etc. will be dislocated, and our aliens have no chance to keep track of that with their cruddy 1945 technology. Every motorway is an airstrip. $\endgroup$ – Karl Oct 4 '16 at 18:53
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    $\begingroup$ @enderland If the aliens have more planes than we have bullets, then we lose. Wired says that the US alone produces 10 billion bullets a year. Let's assume the US accounts for 10% of worldwide production, and let's assume we have 10 years of stockpile internationally. Then let's guess that it takes 1000 shots to down a plane. Then the aliens would need 1 billion planes before we run out of ammo. The whole of WWII managed around 800,000 planes. Then how do you transport 800,000 planes...? $\endgroup$ – Graham Oct 5 '16 at 10:44
  • $\begingroup$ You missed that the OP's scenario accounts for that - if it takes trillions of invading soldiers, assume the enemy has trillions of invading soldiers. They just keep coming until we are subdued - dumping more and more planes until we run out of ammunition, a brutal, massive war of attrition. And even though holding air superiority is a massive boon to a land army, it doesn't make it useless - especially in urban fights. And it's not like modern air fighters cally a lot of ammunition - they will have trouble maintaining continuous air superiority. And every missile we fire is expensive. $\endgroup$ – Luaan Oct 5 '16 at 11:13
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    $\begingroup$ @Luaan It has to be said, I think that's probably the weakest bit of the questions premise. If you have literally infinite troops (and with them, therefore, an infinite supply of weapons) then even a medieval army could eventually overrun everyone if they can deploy quick enough. $\endgroup$ – Dan Oct 5 '16 at 11:21
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The previous answers have addressed

  • How many troops earth has
  • How much danger space debris would pose to the alien fleet.

I will address what force ratio will give a WWII era force equal footing with a modern force. This analysis will assume no nuclear weapons are used (this fight will happen after earth has exhausted its nuclear arsenal destroying aliens)

For infantry there is little difference modern rifles and body armor are a bit better but the difference is likely less than 3:1.

For sea power, the ratio is far worse -- there are 3 attack vectors that a WWII force has no good counter for:

  1. Modern Nuclear subs can stay underwater for weeks and are too quiet to be detected with the low sensitivity sonar available during WWII. They could strike with threat of retaliation.

  2. Modern fighter bombers are both jet powered and stealthy (the are very hard to detect on radar). WWII saw jet powered jets and radar but their radar is not up to detecting a stealthy plane and their jets are not manoeuvrable enough to dog fight against modern jets. Modern aircraft could strike with impunity against enemy aircraft carriers and support ships. There are two limiting factors the WWII aircraft could strike the launch site for the modern aircraft, and the modern aircraft will run out of missiles and fuel at some point.

  3. Cruise missiles allow small destroyers to strike large enemy ships from well beyond the horizon and WWII ships have no counter. The only response is trying to attack the launch ships before they launch or running them out of missiles.

Between these I think a WWII force would need 200:1 odds to take a modern fleet.

The ratio would be as high for air power.

The aliens would be unable to target modern stealth aircraft but they could target the airbases is a massive wave attack even if heavily outnumbered they can still take the airbase. (The initial few waves would be wiped out by airburst nukes.) If the aliens know where our airbases are when they arrive then the odds in the air are not as bad as they are in the sea, maybe only 100:1 after nukes are used up.

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    $\begingroup$ Add that their fleet will be detected years before it reaches earth. They are either coming in very slowly, or they will have to decelerate a lot. Either way, it's Christmas tree in every IR telescope. The whole planet will be in panic and arming. Scale WW2 weapon production by a factor ten? Poor aliens. $\endgroup$ – Karl Oct 4 '16 at 0:10
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    $\begingroup$ The air power ratio is even much worse for our aliens. Radar-equipped and computerised antiaircraft guns would completely wipe out any attempted attack at zero own loss. 1000:0 $\endgroup$ – Karl Oct 4 '16 at 1:28
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    $\begingroup$ @Karl Throw enough shit at the wall and some of it will stick. The AA guns will run out of ammo eventually. $\endgroup$ – Taemyr Oct 4 '16 at 10:12
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    $\begingroup$ While WWII infantry is a serious threat to today's infantry, armored fighting vehicles have improved tremendously. A modern main battle tank could operate with near impunity on a WWII battlefield, though it would probably be lesser units like Bradleys which rack up the most kills. Even the ubiquitous RPG would be overkill against WWII AFVs. $\endgroup$ – Deolater Oct 4 '16 at 12:51
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    $\begingroup$ Even bombing air bases, the US interstate roadway system is designed such that 1 in every 5 miles can be used as a landing strip in times of war. I would imagine we are not the only country to do this. $\endgroup$ – Jammin4CO Oct 4 '16 at 14:47
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Their biggest challenge is going to be the lack of resupply. Traversing the cosmos in chemical rockets will take generations. Even an Orion drive vehicle (which would require access to nuclear technology) will take years to get here from the nearest star, and weeks if they live on Mars. If the unexpectedly run short on something, getting more probably can't happen before it's too late.

Still though, it wouldn't take all that many if they were smart and could pass as either human or mechanized infantry. A few million if they played it right. Land a small force and take over a small, war-torn country that nobody cares about. Use that as a base of operations and a source of actual humans. From there, there are three possible goals:

Stage raids to destroy advanced infrastructure. I've seen estimates that if the entire US power grid were destroyed, 75% of the population would be dead in a few months. Target oil and gas storage facilities as well, and it undoubtedly gets worse. I'd expect that most advanced nations are in a similar boat. They could probably destroy the ability of most first-world nations to wage an effective war before anybody figured out that the attack was extra-terrestrial in origin, especially if they can provoke the various powers into blaming each other.

Second possibility would be "decapitation raids". There are a lot of dictatorships in the world, and nobody looks at it too closely when another warlord comes in and ousts the current one, and the people are used to obeying and fighting for whoever's on top. For the non-totalitarian countries, just be flying the flag of their worst enemy when they charge in and set off their 2KT-equivalent fuel-air bomb (Yes, those were invented during WWII) in the middle of the capitol when the government is in session and everyone's there. In the ensuing chaos, it will be much easier to provoke WWIII (which they sit out in their backwater refuge) and then swoop in and pick up the pieces afterward once most everybody is dead.

Third possibility: Are you kidding? They've got a serious work-ethic, and advanced spaceflight technologies if they can get here. If they attack us, they really can't win without destroying our advanced technology, but there's enough platinum in some asteroids that selling just one would make them able to buy themselves a good-sized chunk of land and access to modern luxury. Set up an ore-processing base on the moon, and maybe conquer that small war-torn nation as a landing zone (although buying it would be better diplomatically) and just start trading. The wealth they could accumulate with this option would far surpass what they'd get after their war of conquest destroys most of the world's capital goods and population, and probably almost none of them would die in the process. (Of course, that does kind of ruin the bit about an invasion from outer space, so probably best to just make them long for a glorious death in combat against overwhelming odds or something like that.)

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    $\begingroup$ The third possibility gets my vote. $\endgroup$ – David K Oct 5 '16 at 17:59
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Much, much less than the other answers suggest. If the aliens are at least a little bit smart about how they do this, I think they could pull this off with an army smaller than the German army during WW2. Why? You seem to be forgetting that the aliens:

Have Nukes

What? I thought this was pre-ww2 technology? Let me explain: The escape velocity of earth is 11.2km/s. In other words, if you drop a sufficiently large and heavy object to earth from rest, it will impact the earth at 11.2km/s (Air friction will reduce this number, but also the ship will probably be moving at a considerable speed to begin with). A 25 ton panzer tank moving at 11.2 km/s will impact with approximately 2% of the energy of the atomic bomb dropped on Hiroshima. This may not seem like a lot, but Germany produced 50,000 tanks during WW2. If the aliens tie 50 tanks in a bundle, and drop them at once, they essentially have 1,000 nuclear weapons. Did the aliens bring a navy with them? 1 capital ship will impact with the energy of 1,000 hiroshimas, which is on par with modern nukes. Assuming they brought 13 million infantry, and each infantry transport weighs as much as the ISS(450 tons) and transports 100 men, then they will have another 65,000 items which can impact with the energy of a Hiroshima.

So, the strategy for the aliens is actually incredibly simple. Basically, destroy the entire surface of the earth, then send in a few million remaining infantry to colonize the earth(hopefully they brought some women), killing off the remaining survivors of Armageddon wherever they are found.

Now, finding the absolute minimum number of troops they would need, basically depends almost entirely on the mass and number of the troop transports, and other vehicles. If the aliens are smart, they won't even bring an army or navy. They'll just bring a million tons of garbage with them.

Edit: I just realized that the question specified "not mass genocide." Nonetheless, I don't think this really changes the answer. All the aliens have to do is nuke a few cities, and negotiate a surrender. Then just keep a bunch of stuff in orbit to drop on earth if earth ever rebels.

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  • $\begingroup$ No problem. As soon as they try to maneuver than "nuke" out of orbit, we send an ICMB that blows all he huge propulsion system they have attached to it into smithereens. One great fireball, and we have a new "moon" afterwards. Smaller items don't work well, as they cant aim with 1945 tech. And Kessler syndrome will also disable those. $\endgroup$ – Karl Oct 4 '16 at 5:40
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    $\begingroup$ I believe this was ruled out in the question "On principle of honor, they refuse to perform acts of mass destruction or non-traditional warfare, such as gas attacks, atomic warfare, or accelerating large objects into the surface of the Earth." fine its many smaller objects but it is mass destruction $\endgroup$ – sdrawkcabdear Oct 4 '16 at 7:40
  • $\begingroup$ @sdrawkcabdear Yes, you appear to be correct. However, I can't possibly imagine that an alien race would rather lose trillions and trillions of soldiers transported across light years, than drop a few thermogravitational superweapons. $\endgroup$ – Salmoncrusher Oct 4 '16 at 14:47
  • $\begingroup$ You make it sound like it's either lose trillions of soldiers or nuke the planet. There is a third alternative: they could go somewhere else. $\endgroup$ – David K Oct 5 '16 at 5:18
  • $\begingroup$ @DavidK Unless our advanced technology is precisely why they are attacking. Maybe the empire is in WWII technology stasis, and they need our technology above all else. That would also give a good reason not to destroy the Earth's surface and our population - they need the cities, factories, engineers... Ultimately, they want us, not our planet. $\endgroup$ – Luaan Oct 5 '16 at 13:28
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Actually there is a series of SF books by Harry Turtledove almost covering your topic. https://en.wikipedia.org/wiki/Worldwar_series

The only minor difference is that the invading aliens have slightly better warfare technology (like from 1990) than humans during WW2.

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    $\begingroup$ This is also similar to the Posleen stories by John Ringo. There the tech is a bit more advanced but still fairly equivalent with what we have today. The real kicker to this scenario is that the aliens have no real supply chain (if they did then why bother fighting for an occupied planet when they could just take Mars?) so that puts them at an EXTREME disadvantage, even more so when there is tech parity (or an edge for the humans). $\endgroup$ – Jason K Oct 4 '16 at 13:50
  • $\begingroup$ There's also a major difference - the OP wants the aliens to win, no matter how many (alien) casualties it takes :) $\endgroup$ – Luaan Oct 5 '16 at 13:31
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I think they can "win" fairly easily with an army in the sub-billion range, provided the earth doesn't have a few years warning to re-arm beforehand. We can't retaliate against their cities, and it becomes difficult to use the nukes once they've landed. They'd have to start by paradropping near to urban areas and taking over cities first.

Logistics and supplies will be crucial, theirs and ours. Just as in WW2 oil was critical, so it will be now. Shipping is also crucial. Both of these are concentrated in a small number of unprotectable supertankers and supermax container vessels.

If they need to capture our oilfields to keep going, that will determine where the battles are. If they don't, that's a massive advantage to them.

You've mentioned that they don't intend genocide, but I don't think they can win without seriously damaging the global infrastructure that keeps 7 billion people fed and warm. There's going to be worldwide waves of refugees while at the same time shipping food internationally becomes much harder. Things like the natural gas pipelines that keep Europe warm in the winter will be destroyed.

Also, I don't think that you can assume that humanity is going to present a united front. And we've already seen how unenthusiastic people are about refugees; if faced with the choice of "your food is going to be rationed to starvation levels so we can feed the arriving millions" versus "sink the boats", plenty of people will advocate murdering the refugees.

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  • $\begingroup$ Wouldn't "food in starvation levels" lead to death by starvation? $\endgroup$ – nuoritoveri Oct 5 '16 at 12:38
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First let me say that nuclear weapons would not be able to destroy enemy ships. The ship is capable of surviving speed faster than the speed of light then I will definitely survive a nuke.

Second let me say that World War II military and weapons will not stand a chance against our military weapons. Not only do we have better weapons we have better communication assuming that they're still relying on World War two methods of communication then there's probably very little communication between the ships and the enemy forces and if there is there was a gigantic time delay. Whereas our forces can communicate with are satellites in real time.

Ultimately I see this war ended in a draw. We would not be able to damage their ships but they would have been if they run out of ammunition for them I would not be able to continue firing. We would win the ground war. So ultimately it would be in the place will we would not be able to hurt them but neither would they be able to hurt us.

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    $\begingroup$ That's not what the OP said. "minimum necessities" and "crude methods". Sounds to me like chemical combustion engines and travel times of thousands of years. And nothing that is not highly sci-fi can repel a nuclear explosion. $\endgroup$ – Karl Oct 4 '16 at 1:05
  • $\begingroup$ The question said that the civilization would have only the bare necessaries for space travel faster-than-light capabilities isn't necessary for space travel. Since these people obviously don't live in our solar system they must have come from someplace else to do that they will need faster-than-light capabilities $\endgroup$ – Bryan McClure Oct 4 '16 at 2:49

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