The alien invasion is a common theme in science fiction stories and film, in which extraterrestrials invade Earth either to exterminate and supplant human life, enslave it under a colonial system, harvest humans for food, steal the planet's resources, or destroy the planet altogether.

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The invasion scenario has been used as an  for a protest against military hegemony and the societal ills of the time. H.G. Wells' novel The War of the Worldsextended the invasion literature that was already common when science fiction was first emerging as a genre.   The most well-known alien invasion scenarios involve the aliens landing on Earth, destroying or abducting people, fighting and defeating Earth's military forces, and then destroying Earth's major cities. Usually, the bulk of the story follows the battles between the invaders and Earth's armies, as in The War of the Worlds. However, not all alien invasion stories follow this plot. In some accounts, the alien invaders will covertly subvert human society using disguises, shapechanging, creating conflict in humanity and let humans destroy themselves, or human allies. In other depictions, the aliens score an overwhelming victory over humanity and the bulk of the story occurs after the aliens have taken over. Sometimes, the aliens do not come from space, but from another dimension, as is shown in the 2013 film Pacific Rim. And in some fiction, the invaders may not actually be aliens, but demonic creatures.