Alien invasion
Alien invasion is a science fiction theme in which extraterrestrial life attacks Earth with the intent to conquer it. The classic treatment was The War of the Worlds by H.G. Wells.
Other treatments have posited biological invasions (Invasion of the Body Snatchers), or cultural invasion (The Uplift Wars by David Brin).
Alien invasion was a common metaphor in science fiction during the Cold War for the fears of the American people.
Alien Invasion -- Alien Invasion