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Scenario: an alien race has traveled across the gulf of time and space, let's say from the Alpha Centauri system, fleeing their own dying world and looking for a suitable replacement. They find out about the earth as they enter the solar system.

Now, obviously if these guys are capable of interstellar travel, their technology is far beyond what humans have. But there are two limitations:

  • they have the tech, but not the numbers. Keep in mind that these guys are essentially refugees, tired and exhausted from their long journey. Maybe there are a few thousand, maybe tens of thousands, but there are way, WAY more humans.

  • they need the earth to be in more or less intact condition for them to recolonize it, so dropping an asteroid onto it or nuclear weaponry is not ideal as they would have to wait a long time for the planet to recover - they'd rather start the colonization process now. Let's say that they used up all their resources getting to this point and they really can't wait around for the years it would take for the earth to recalibrate.

What can the aliens do to cut down on the human population? Perhaps some kind of bio weapon that kills most people? This way the aliens can then put boots on the ground and just finish off the rest?

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closed as too broad by colmde, Frostfyre, We are Monica., Renan, Mephistopheles Oct 21 at 16:19

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Star wars might be able to help $\endgroup$ – LiNKeR Oct 17 at 23:16
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    $\begingroup$ See my answer to a different question here:worldbuilding.stackexchange.com/questions/158239/… Not everything is relevant, but TLDR: Fleeing into another solar system if you do something bad to your planet is quite stupid (if you can build an interstellar ship, you can also build arcologies and O'Neill cylinders much more easily), our biosphere would be worthless to them, and they would have no problem annihilating us. $\endgroup$ – Richard Smith Oct 18 at 13:33
  • $\begingroup$ Mass-produce Marylin Monrobots. $\endgroup$ – Headcrab Oct 21 at 6:28
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    $\begingroup$ Not sure what the dilemma is here - you say they are "technologically advanced" so you're limited only by your imagination... making this question a sort of request for ideas... $\endgroup$ – colmde Oct 21 at 8:49
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    $\begingroup$ Hibernate and wait till the humanity destroys themselves? $\endgroup$ – Tomáš Zato Oct 21 at 8:56

15 Answers 15

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This has been done many times. The European colonisation efforts in India, Africa and Polynesia amongst others are very well documented, but many outfits did the same.

Basically you provide weapons and perhaps training and expertise to one side to defeat the other in their own interests. Then pick a reason to wipe out the first. Or just arm the remnants of the defeated and let them do it.

Even easier these days as there are all sorts of extremist groups you could arm and let loose.

The thing about humans is they're quite happy to wipe each other out if given a chance.

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    $\begingroup$ Also - smallpox helped the Europeans. The aliens can also use a bio weapon that they are (mostly) immune to. $\endgroup$ – VLAZ Oct 18 at 8:50
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    $\begingroup$ @VLAZ, the alien common cold would probably work just fine! $\endgroup$ – EveryBitHelps Oct 18 at 15:52
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    $\begingroup$ Why not start a war and secretly arm both sides? Preferably with something you can later use to your advantage. +1btw $\endgroup$ – Joe Bloggs Oct 18 at 16:41
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    $\begingroup$ @EveryBitHelps, actually no. Europeans and the Natives had a very recent common ancestor. Aliens will have an entirely different biological system. They probably won't even have the same nucleotides as humans. $\endgroup$ – Aravind Karthigeyan Oct 19 at 23:56
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    $\begingroup$ How did Cortez conquer an entire empire with only a few hundred soldiers? He had lots of native allies, who decided that they would rather live under Spanish rule than become human sacrifices for the Aztec rituals. $\endgroup$ – vsz Oct 20 at 8:24
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Don’t drop one asteroid.

Drop thousands

The mathematics of impacts is fairly well understood (though it is a little fuzzy at high speeds). For any given weight and shape of ‘asteroid’ (really an aerodynamically shaped, ceramic coated lump of rock with some engines attached) hitting the ground at any given speed: You can quite accurately predict the blast yield and crater penetration.

Not only that, but with the power of computers, automation and orbital mechanics you could time the release of all your projectiles such that they impact within the same (very short) window.

This lets you tune the devastation you wish to rain down very precisely, letting you obliterate even hardened enemy military, industrial and infrastructure centres. Do it right and humanity won’t even know you’re there before the first wave reduces all of their hopes to rubble, permanently forestalling any hope of a counterattack. As all the individual impacts are low energy and quite spaced out (comparatively) any dust they throw up will settle quickly and areas of concern (like those big ol’ rainforests) will be spared destruction. Do your best to minimise the number and size of impacts required by sparing large population centres: without government or food supply they’ll turn into more of a burden to humanity than a help.

After that you send in the ground troops. If they meet resistance: Drop more Rods From God. Rinse and repeat as necessary.

Basically: If you control orbit and can manufacture the ceramic coating you can keep throwing just-destructive-enough asteroids with no fear of repercussion. Humanity has no defence against that.

Plus the attack diagrams would look amazingly cool.

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    $\begingroup$ Only problem with this is it could trigger an automated nuclear response, depending on what's happening politically on the ground en.wikipedia.org/wiki/Dead_Hand $\endgroup$ – Riddles Oct 21 at 8:19
  • $\begingroup$ @Riddles: Not if you’ve already dropped a rock on all the nuclear capable sites. Subs might be a touch trickier to hit in the first wave, but if your alien super tech can spot them through the ocean you could probably just pepper the general area with rocks and let the impact pressure waves crack the subs open. $\endgroup$ – Joe Bloggs Oct 21 at 11:52
  • $\begingroup$ @Riddles The technologically advanced aliens will presumably have better missile interception tech than we do. $\endgroup$ – ceejayoz Oct 21 at 13:47
  • $\begingroup$ I was thinking more firing and irradiating large areas, not shooting back. "they need the earth to be in more or less intact condition " $\endgroup$ – Riddles Oct 21 at 15:05
  • $\begingroup$ @Riddles: fallout will be much less of an issue if most of the nuclear material is buried in a rubble filled crater or still stuck inside the warhead of an otherwise disabled ICBM. Plus these guys have managed to deal with hard radiation all the way from wherever they’ve come from, so a relatively small amount of cleanup around the silos they hit wouldn’t be unthinkable. $\endgroup$ – Joe Bloggs Oct 21 at 16:29
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Engineer a virus specifically targeted to kill humans which is very contagious and has a long gestation time, say 6 months. Introduce it into various cities around the world and let it spread invisibly. Wait 6 months and watch the majority of the human world population vanish. Sprinkle it on the remaining patches of human settlement to finish the job.

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    $\begingroup$ Better yet, use a new virus the second time in case some people have immunity to the first one. $\endgroup$ – Klaus Æ. Mogensen Oct 18 at 8:13
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    $\begingroup$ This may even be within the current reach of humans, or soon will be. $\endgroup$ – James Reinstate Monica Polk Oct 21 at 2:00
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Unless you're using some form of handwavium drive you have some incredibly powerful energy emitter on your starship--remember the Kzinti lesson. For your ship to have made it across the stars without being a slowboat it must be expelling something at high relativistic velocity. If it's any sort of particle simply point the drive at the planet--fry every city and town. While you won't get a 100% kill you'll get enough to crash civilization without tearing the ecosystem up too badly (you're only frying urban and suburban territory--your drive is certainly highly directional as the wider the spread of the particles the more energy was wasted.)

A photon drive below x-ray energy won't be able to irradiate people like that but you can still use it to cook cities.

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    $\begingroup$ Nice idea. Even if the effects are too slow to work on people just systematically frying all electronics from the top 1000 cities would make the earth a "not nice place" for humans. So what if the military hardened systems remain in place They have finite resources if they are not fed and watered by civil society. $\endgroup$ – KalleMP Oct 19 at 13:14
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    $\begingroup$ Upvoted for the Kzinti Lesson. Classic. $\endgroup$ – Corey Oct 20 at 22:41
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Why kill useful workers?

Humans are herd animals, it is relatively easy to keep them inline. Especially if most are unaware that they are being kept inline.

The easiest approach would be to gain trade access, followed up by enlarging their own population while simultaneously causing attrition to the indigenous population. The last step would be complete integration or annihilation.


To that end, they would land somewhere relatively innocuous, but with a handful of pliant, charismatic, and socially acceptable individuals. A quick analysis of the planets primitive communication network should reveal any number of such individuals.

These individuals are provided with information and designs well in advance of anything available in this primitive backwater, but still quite old tech, or perhaps some insight into the market that could be passed for believable intuition. This gives a gateway for aliens to trade with the local population (via these individuals/companies) gaining access to the planetary workforce, and production.

Having access to the market provides the means to replenish, repair, and rebuild what the refugees had lost. This gives the simplest access to processed goods such as food, water, fabrics, and raw resources. If the right businesses were established this can also give access to the industrial machines responsible for refining, shaping, and assembling.

Being able to source even relatively primitive manufacturing capacities can serve to relieve load off the limited manufacturing capacity the refugees still have. Even primitive materials can have useful applications. If the refugees lost their own ability to manufacture, this is even more important, as it will allow them to cobble together manufacturing capabilities in order to repair their ships, or expand with other necessary infrastructure projects that cannot be built by the indigenous humans.

After sometime there exists the possibility that the selected individuals become too risky - accidents can be arranged. By this point enough local customs have been learnt to train several special operatives. Through some form of remote control, reconstructive surgery, or holographic imagery they can pass as human. So replacing the individual with a "foreign business man" isn't too unreasonable.

Over the next few years the company goes from strength to strength. It is now generating revenues comparable to several small countries and large states. Time to get into politics. Bribery, thought policing, back room deals, advertising, and even outright campaigning to install suitable leaders. Perhaps even start wars between several nations which are already in conflict. Perhaps even influence the culture to support destructive cultural norms to reduce their overall birth to death rate.

By the time the humans are even aware of the alien presence it is already too late. Either most of population are alien/human hybrids, or the aliens significantly out number the humans, or there is no way for the humans to out compete the aliens and while they may not be defeated, it is only a matter of time.

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  • $\begingroup$ I like your initial idea but I quibble on if humans are herd animals, this suggests that they evolved as prey (think sheep, cows and horses). I think evolutionary we are predators and as a result we are pack animals, (think wolves and lions). $\endgroup$ – Burgi Oct 21 at 16:34
  • $\begingroup$ Brilliant idea. I want to play that game! $\endgroup$ – Martin Grey Oct 22 at 18:28
  • $\begingroup$ @Burgi The truth is the middle ground. We were definitely herd animals in that we were actively hunted by a wide array of carnivores, to the point were some felines had adaptations that arguably made it easier to hunt hominids. Yet at the same time hominids also invented the persistence hunt, and more direct hunting techniques themselves. But, when I mentioned herd animal, I was actually indicating the ease with which influencing the leaders in the group confers control of the group. Most humans will comply with an order (however distasteful) when given through a leader. $\endgroup$ – Kain0_0 Oct 23 at 23:10
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Apply directed thermal changes to key areas of the ocean.

We already know that climate change can have an impact on global food sources through changing the way food is distributed throughout the oceans, and the impact that has on land dwellers. In point of fact, this isn't so hard to disrupt because most of the world's food supply is reliant on the Global Conveyor Belt, a large set of only a handful of deep ocean currents.

With a good understanding of how these currents work, and enough energy to superheat a large region of ocean (say, a couple of square kilometres) over a sustained period of time, you could cause mass starvations in less than a year. That in turn would lead to global conflict as nations start to fight for other sources of food security, and before long humans have done your job for you.

This is still a long term solution insofar as you will want it to play out over a couple of years, but asteroid bombardment or nuclear detonations will take decades, if not centuries, to clear so this is still a relatively quick answer given your immediate need. Also, the singular benefit is that you don't even have to reveal yourselves to do it; you can find a remote area of the ocean to heat, and your superior stealth tech means you can remain undiscovered while doing it, and the humans just think it's accelerated climate change impacts and blame themselves for it all in any event, rather than looking up and saying 'who the hell is up there throwing asteroids down on us?'

The other advantage to all this is that you lose a lot of life around the planet, but you don't actually lose the biomass. This means that you can turn off your water boiler, put your own fish and food supplies in the water and on land and there are still plenty of organic molecules out there that your species and those from your planet you wish to see colonise the earth can co-opt for their own growth into the biosphere.

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Cyberwarfare against transport infrastructure

Assumption 1: The aliens have monitored broadcast transmissions and have learnt our language and technology

Assumption 2: Unlike the Independence Day aliens, these aliens are well ahead of Earth in cyber warfare technology including cryptography. It is not unreasonable to expect that they have fully functional quantum computers which can trivially break Earth's existing public key encryption protocols.

The aliens remain well away from Earth and use drones to run communications interception and spoofing. They crash the world's financial system and satellite navigation systems, black out as many power grids as they can and insert creatively bad data through government and corporate communications networks to interfere with supply chains of necessities. For maximum impact this should be timed to coincide with extreme weather conditions across as much of the northern hemisphere as possible - heatwave or massive snowfall will both work well.

Modern cities are not remotely self-sufficient - within 24 to 48 hours of cutting off the organised supply of food, water and electricity there will be chaos. Expect 95% die off of the urban human population within 3 weeks with minimal adverse environmental effects.

At this point the aliens can move into Earth orbit, eliminate all remaining human-launched satellites at their leisure, then conduct orbital reconnaissance to select an isolated colonisation site on Earth. Invade and eliminate all humans in the area, jamming any HF transmissions. The surviving humans on the rest of the planet will not even know that the landing has occurred.

The aliens maintain a strong (but lightly manned) orbital presence with a good supply of rocks, eliminate anything the humans try to launch up the gravity well and drop appropriately sized rocks down the gravity well to deal with any human settlements recovering faster than they like. Once the alien colony is ready it can invade another area. With control of the orbital high ground, the aliens have full knowledge of what the humans are doing while the humans are fumbling around in ignorance.

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Sow chaos on multiple, progressive levels

It must be a multi-layered campaign and happen sequentially.

1. Ideologies

Insert ideas into society on all sides of every issue, radicalize socio-political ideals so that every idea seems genuine, but makes zero room for them to have compromise, cooperation, progress, or understanding of what divides them.

2. Infrastucture

Injure infrastucture just enough to create societal chaos and dissent. The goal is not to prepare for warfare or invasion, just enough to create trouble. Then an already divided people will fight each other more, blaming each other for not responding better.

3. Deny governance

As governments and authorities try to solve problems and bring peace, stop them, "disappear" them, and otherwise inhibit those who could restore the social order.

4. Arrive to deliver

Once society, infrastucture, and peaceful governance have failed, arrive as the saviors. The people will welcome you as an improvement to the status quo's chaos. Even though your plan is to somehow conquer, being welcomed by the people you wish to subdue will make your efforts a breeze.

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This is a pest control scenario: From an alien's perspective, there is an unwanted species all over the place, and you want to dispose of it with minimal effort and minimal side effects.

Distribution is not too difficult: Just deliver your toxin in respirable dust grain sizes.

There is a wide choice of toxins; I can think of: Any alpha emitter (takes a while to act but humanity can do little about it); chemical poisons (not sure whether rizin would work, but I bet you can formulate nerve gases so that they are bound to a dust particle and set free as soon as they attach inside the lung); parasites; bacteriae and viruses.

The aliens would abduct a few specimens, try out different agents, select whatever needs the least amount of material while having the least amount of unwanted side effects.
Distribute the stuff in the upper atmosphere, see whether the death rate goes up enough to kill civilisation. If it does not, try the next agent - those earthlings can't touch our vessels, so we can afford to experiment.

Ignore those remaining humans, just squash them whenever they become worrisome.
If you need labor, you can easily place them at whatever level you want them, from slave to advisor, both individually or as a group.

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I'm surprised nobody has mentioned the neutron bomb.

In a neutron bomb, the casing material is selected either to be transparent to neutrons or to actively enhance their production. The burst of neutrons created in the thermonuclear reaction is then free to escape the bomb, outpacing the physical explosion. By designing the thermonuclear stage of the weapon carefully, the neutron burst can be maximized while minimizing the blast itself. This makes the lethal radius of the neutron burst greater than that of the explosion itself. Since the neutrons disappear from the environment rapidly, such a burst over an enemy column would kill the crews and leave the area able to be quickly reoccupied.

It would take a lot of them, but presumably that's not too difficult for an advanced alien society.

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  • $\begingroup$ Not a bad first post, welcome to the site. +1 (From review). $\endgroup$ – We are Monica. Oct 21 at 15:28
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For some reason the following script comes to mind:

  1. To want the Earth's biosphere largely in tact the aliens must really love most of the local biochemistry. Most likely their own must depend on DNA also (ripping off E.T. here).
  2. Therefore their weapon (like many others I would use a biological one) could exploit a very subtle difference in biochemistries of the two planets, or their own and that of humans in particular. I might suggest some form of stereoisomerism. May be a chemical that on Earth only appears in D or L-form? The aliens and most lifeforms on Earth being indifferent to the difference, but for some reason the humans find the wrong form poisonous. Entering handwavium-territory here.
  3. To figure all this out the aliens obviously need a bit of time to collect intelligence. This means that they need to be very few in number to remain undetected in an outpost. I will shamelessly reuse the idea from The Invasion of the Terrible Titans by Sambrot William. The aliens were able to survive in lower atmospheric pressure and (relatedly) handle a higher level of ultraviolet radiation. Hence they set up their base in the Himalayas to remain largely undetected. IIRC in that short story the aliens ended up playing NCAA football instead of conquering the Earth, but my recollection may be a bit off, and I don't have the time to check my copy :-)
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dropping an asteroid onto it or nuclear weaponry is not ideal as they would have to wait a long time for the planet to recover

Yes it's ideal, and no they wouldn't have to wait.

First off, it's not going to "damage the Earth". Even one dinosaur-killer asteroid would only make one fairly small dent which would be indistinguishable from a natural volcano. Of course that'll mess up the atmosphere for a bit though,, so let's not do that.

Instead, hit the Earth with barrages of smaller asteroids, large enough to get through the atmosphere but small enough to "only" have the impact of a MOAB. You can saturation-bomb all inhabited regions (hint: just aim for any part which has lights on at night) and ensure you're landing a rock every hundred yards. Or don't even be that accurate - just keep throwing rocks continuously for a month with enough random scatter, and you're going to do the job.

There's still going to be plenty of dust in the air, of course. Give it a few months to settle though and your aliens will be fine. Volcanoes kick off all the time and cover their local area with dust clouds, but it doesn't take long for the dust clouds to clear.

Simple really.

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  • $\begingroup$ One light, one rock! Simple and efficient rule :) $\endgroup$ – Madlozoz Oct 21 at 12:39
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Pretty much anything they want

I'll give you GATE as an example. Portal to a fantasy world opens up in the middle of Tokyo and while a lot of havoc is caused immediately and hundreds of people are killed, the moment the Japanese military mobilizes and gets boots on the ground, they set up a defensive perimeter on the fantasy side in about a day. The nearby kingdom eventually sends its army over to attack what it sees as an invasion and gets thoroughly slaughtered because they sent swordsman and cavalry against machine gun emplacements. The JDF probably had only a hundred or so men manning their encampment and the kingdom's army was measured in thousands (the JDF saw no losses, as not a single man from the massive army made it within 400 feet).

The JDF eventually has an encounter with a dragon and while it shrugs off a lot of the munitions thrown at it, they eventually blew off one of its forelegs with a portable rocket launcher (then tracked it down to its lair, set up C4 charges, and blew it up). Oh, and the team that was dealing with this problem? Four. It took just four guys (in a jeep) to defeat a dragon that was shrugging off anti-vehicular machine gun rounds.

Humans to aliens with interstellar capabilities are going to go down much the same way. "Why don't we drop a nuke on them?" well, because insert technobabble here and their whatever tech stops it. Maybe the aliens have energy shields, maybe they can detect nuclear warheads hundreds of miles out and shoot down the bomber, maybe they've got some kind of containment device. Whatever fits your bill.

The aliens being tired from a long journey is almost irrelevant when they can field one man for every ten thousand of humanity and still call it a fair fight. Especially when they can land, set up base, and not even really be noticed by the local authorities for a couple days. Even then they'd just have to deal with the hundred or so local police and then have a week before anything significant (the military) shows up.

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  • $\begingroup$ GATE, people from Japan have supply line (the portal still open). If the portal close for good, troop who trapped without supply will be defeat in the long run (if the king are smart,... really) $\endgroup$ – Haha TTpro Oct 20 at 14:32
  • $\begingroup$ You're not wrong overall, but if a country with a modern military took interest they would show up with planes in much less than a week. Which would presumably be immediately shot down. $\endgroup$ – nasch Oct 21 at 15:12
  • $\begingroup$ @nasch You have to keep in mind the delay time between the first observation and the military recieving orders. The airforce can't just bomb a place without authorization, which requires (at a minimum) presidential approval of not a full vote of the Senate. $\endgroup$ – Draco18s Oct 21 at 18:00
  • $\begingroup$ Well I didn't say anything about bombs. If the US military, for example, took an interest in something that looked like it could be a threat, they could have some kind of plane or drone in that spot in less than a day if they wanted to, anywhere in the world. Whether there would be an attack launched is another question. $\endgroup$ – nasch Oct 22 at 1:27
  • $\begingroup$ Sending someone out to do recon (someone can be "a drone") fairly early on, sure. But I wasn't talking about "someone doing recon" I was talking on the level of "showing up with tanks." Because "a recon drone" is not "anything significant." $\endgroup$ – Draco18s Oct 22 at 3:04
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Do the politics, make some human see the "profit" to work for you. Then that group of human will destroy, enslave other who refuse to coop.

XCOM (human defeated), XCOM 2 is good example.

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Do it like they did in the movie: The 5th Wave.

https://www.imdb.com/title/tt2304933/?ref_=fn_al_tt_1

This sounds exactly as the same premise as in the movie The 5th Wave.
As you can probably guess the aliens in that movie did it in 5 waves.

1st wave, an electromagnetic pulse that permanently disables all electrical power grids worldwide.

2nd wave, manipulate the planet's geology and fault lines, causing earthquakes and megatsunamis that destroy coastal cities and islands. You could also do this using asteroids.

3rd wave, modify a strain of avian flu virus and spread it across the planet with birds.

4th wave, pretend that you are the military and gather up all the children you can find and train them to be soldiers against the alien invaders. Tell them the aliens are masquerading as humans, but you have the tech to detect them.

5th wave, unleash the child soldiers on the remaining population.

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