The Day the Earth Stood Still movie

A flying saucer orbits the 1950s Earth. It lands in Washington, DC, on the Mall. The lone occupant steps out and is shot by a jumpy soldier. Gort, a large and very powerful robot appears to save him and is able to melt tanks with the slightest bit of his power. The wounded alien orders Gort to stop his rampage and is taken to a hospital from which he escapes in order to learn more about this planet, even moving in as a boarder with an Earth family. When they begin to suspect him, he reveals himself, along with the news that Gort is a member of a race of super-robot enforcers invented to keep the peace of the galaxy that will destroy the Earth if provoked.