The Day the Earth Stood Still (1951)

Unlike most of the invasion pictures around it, this film opens not with death rays but with a silver disc settling quietly onto the National Mall in Washington. Its passenger, Klaatu, looks entirely human; beside him stands the silent, near-omnipotent robot Gort. He comes not to conquer but to warn: if humanity takes its nuclear violence into space, Earth will be removed. Arriving only a few years after the 1947 sighting wave gave America the phrase 'flying saucer,' the film helped fix the disc as the default shape of the UFO in the popular mind, and pioneered the alien as messenger rather than monster.^[2]