Alien Invasion
Alien Invasion refers to a move by extraterrestrial civilization to conquer earth. A staple of science fiction since HG Wells, it is also a possible doomsday scenario.
Real Threat
Considering the sheer scale of our galaxy, let alone the universe, it is quite likely that Evolution will have produced life on other planets, and some of that life might evolve cognative copacities rivaling our own. A civilizatation from a sapient species older than ours could conciviably develop a method of crossing the stars (such as puting the crew of an antimatter pulse detonation powered ship into suspended imagination and have it fly across intersteller distances at a decend fraction of the speed of light) allowing them to spread themselves across the universe and if they come this way, they could easily destroy our civilization via orbital bombardment and set up their civilization over the remains of ours or subjugate ours with the threat of orbital Annihilation, gradually displacing our own. In most conceivable scenarios, humanity would be unable to resist an assault by an alien conquest flet.
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