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My current story is post-apocalyptic, but instead of a virus/asteroid/zombies, I'm going for an alien-caused catastrophe.

The setting is a future Earth affected by depleted resources, climate change and conflict between major world powers. The planet is teetering on the brink as it is, and the midst of all this chaos, the alien ships arrive.

I'm trying to figure out what could possibly cause the aliens to attack Earth, leading to a war between the two species and an eventual post-apocalyptic planet with the the remnants/survivors of both species duking it out for ultimate survival

Maybe these aliens are actually fleeing their own dying planet, and the visitors to Earth are actually the last survivors. However, they came prepared with weapons and armaments just in case and used up the last of their fuel to arrive on Earth. They want sanctuary and shelter with Earth being the only habitable planet around. But Earth can barely support itself, let alone millions of new alien inhabitants (they arrived in generational arkships). But the aliens need to ensure their survival at all costs and tensions arise and war breaks out. Humans have greater numbers but the aliens have way better tech so it ends in a zero-sum game as they blow each other to bits

Is this a possible reason an alien race would wage war against humans?

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    $\begingroup$ Well, think of us humans that encountered other humans when going to new islands, continents, etc. We've done pretty much what you ask here with very little justification needed. Mostly greed (locals have resources) and prejudice (they are savages, so nobody cares). Aliens can have a similar view when discovering Earth. $\endgroup$
    – VLAZ
    Oct 3, 2019 at 6:24

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You need some kind of plot device to explain how aliens advanced enough to cross the stars wouldn't simply wipe out humans with their overwhelming technological superiority.

You could make the aliens escaped refugees on the losing side of their own war (with another faction of their race, for example). This would justify the aliens showing up a bit unprepared, and unable to simply wipe out all humans at the press of a button. Maybe their ships are not even warships, but merely rickety transports with some clumsy weapons attached (but still advanced enough to outclass human weapons). Perhaps they do not have professional soldiers with military equipment, but only small arms meant for self-defense. Even with advanced technology, the aliens might have very limited resources left, and be forced to use them conservatively to deal with humans.

This backstory would also justify the aliens not calling in any reinforcements, as they are fleeing/hiding from pursuers who are much deadlier than humans. Perhaps they are just desperate to acquire the resources from Earth to produce more ship fuel, so they can pack up and run away even further.

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    $\begingroup$ Good, but I would add one thing - 30 ky, when they started their expedition Earth was an icy planet with no detectable civilisation, so taking anything more powerful than rifle was being considered as total waste of fuel. $\endgroup$
    – Shadow1024
    Oct 3, 2019 at 6:12
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    $\begingroup$ @Shadow1024 nice +1 for sub-light speed interplanetary surveying as an explanation for ill-prepared aliens operating on outdated information. $\endgroup$
    – vinchenso
    Oct 3, 2019 at 6:44
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    $\begingroup$ Maybe the aliens are very well prepared to exterminate the indigenous life, but they were expecting us to be silicon based and none of their pesticides work against carbon. $\endgroup$ Oct 3, 2019 at 15:48
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Yes and no.

Yes it's plausible that that motive for the aliens could lead to violence.

But it's not plausible that it would lead to war. It would be more like pest control. If you have interstellar travel, the technology gap is just so extreme there's nothing even a united modern Earth could do about it. They could just release their trash on an intercept course with Earth and it would hit with the force of tens of thousands of nuclear bombs. And they also must have highly effective point defense weaponry, because at interstellar velocities, even grains of sand are a threat. Even if we tipped our fastest spacecraft with nuclear warheads, they would never get close. If their point defenses can pick off a grain of sand going 0.1C, they will have no trouble hitting something the size of a sedan going 0.00001C.

Think about this. 168 Spanish Conquistadors defeated the entire Incan Empire, who's armies numbered around 80,000. And that's given only a few hundred years technology advantage. If these aliens are capable of interstellar travel, they must be thousands of years ahead at least. Does that sound like it'll be much of a war?

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    $\begingroup$ They might even see the humans as an annoying local fauna $\endgroup$
    – L.Dutch
    Oct 3, 2019 at 5:10
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Terra Nullius

Ultimately if you are going to take over another planet as either a new home or for expansion purposes, you may just want to wipe out life on it first. Not the capacity for life, just the life that's already there.

For a start, depending on your own biology or organic chemistry, the food and/or environment may be of no use to you, or worse yet, contain certain compounds in high volumes that are toxic to you. If so, you don't want to get rid of the water, the carbon or any of the other elements that make it favourable to life; you just want to use it for your kind of life, not for the native inhabitants.

As such, you may want to simply wipe out the life forms already there without wiping out the environment. If there is intelligent or otherwise dangerous life forms there, starting with an 'empty earth', or Terra Nullius, solves a lot of problems in terms of negotiation and local resource management. You just wipe out what you can, but do so in a way that preserves the raw materials that you can reuse for your own purposes.

So if aliens were to come here, there are strong chances that they won't be here to talk or negotiate treaties or even to eat us; they may well be here to wipe us out and use the materials that used to be... well, us, to rebuild the earth in their own image.

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Why do we always see an alien race as smart and genius and always capable of planetary-scale destruction? I would like to extend your plot to make it more fun and hopefully plausible.

There is an alien 'X' race far-far-away, that is at a brink of breaking out of their available resources, they have the high-end tech, but that is all donated by other brighter and smarter alien races. While these 'X' are not so smart. Now, they have used up all their resources and seeing this no other races are giving them any kind of shelter, So they are desperate for survival.

Now, Humans are constantly sending out signals into deep space so as to find life, While other races know the meaning of these signals and choose not to respond, these Alien 'X' mistranslated the human signals and thought we are inviting them and willing to help.

So, they packed up everything and in a mad desperate hope comes to Earth, leaving all their weapons of mass destruction behind so as to make as much space on their ship as possible for all of them.

That's where all the issues started, but neither they or us were planning to wage war and were actually willing to talk.

But as all stories go, a stupid enough human managed to kill an even stupider Alien 'X' prince, which angered them and the war started. But as they left their weapons behind, and were not smart enough in planetary war tactics, we actually had a fighting chance, and we all end up in a post-apocalyptic world.

I hope, the background story was fun.

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    $\begingroup$ +1 for suggesting less advanced and peaceful aliens. Clearly the peace-loving, space-hulk faring tyranids and space orks of the WH40K setting are merely just misunderstood space tourists. $\endgroup$
    – vinchenso
    Oct 3, 2019 at 6:40

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