The Day the Earth Stood Still

When a flying saucer lands in Washington, D.C. in July 1951, the Army quickly surrounds it. A humanoid (Michael Rennie) emerges, announcing that he has come in peace. When he unexpectedly opens a small device, he is shot by a nervous soldier. A tall robot emerges from the saucer and quickly disintegrates the soldiers' weapons. The alien orders the robot, Gort, to stop. He explains that the now-broken device was a gift for the President which would have enabled him "to study life on the other planets".

The alien, Klaatu, is taken to Walter Reed Hospital. After surgery, he uses a salve to quickly heal his wound. Meanwhile, the Army is unable to enter the saucer; Gort stands outside, silent and unmoving. Read more on Wikipedia.