The Day The Earth Stood Still (1951)
This is the 1951 version, the original and the best, avoid the 2008 remake with Keanu Reeves. TDTESS is not just a good movie yarn, it was also the first sci-fi blockbuster, the first to show flying saucers on screen and contains a mass of state of the art visual effects pre CGI. Bear in mind that this was the height of the cold war, the race to have the most atomic bombs was on, McCarthy & Co were about to start asking whether people were now, or had ever been, practising communists and atheist commies were perceived to be the threat to God, King and Country. What better message than for alien daddy to tell his Earth children to play nice and be nice, or else. Gort! Klaatru barada nikto!
Klaatu: I am leaving soon, and you will forgive me if I speak bluntly. The universe grows smaller every day, and the threat of aggression by any group, anywhere, can no longer be tolerated. There must be security for all, or no one is secure. Now, this does not mean giving up any freedom, except the freedom to act irresponsibly. Your ancestors knew this when they made laws to govern themselves and hired policemen to enforce them. We, of the other planets, have long accepted this principle. We have an organization for the mutual protection of all planets and for the complete elimination of aggression. The test of any such higher authority is, of course, the police force that supports it. For our policemen, we created a race of robots. Their function is to patrol the planets in spaceships like this one and preserve the peace. In matters of aggression, we have given them absolute power over us. This power cannot be revoked. At the first sign of violence, they act automatically against the aggressor. The penalty for provoking their action is too terrible to risk. The result is, we live in peace, without arms or armies, secure in the knowledge that we are free from aggression and war. Free to pursue more... profitable enterprises. Now, we do not pretend to have achieved perfection, but we do have a system, and it works. I came here to give you these facts. It is no concern of ours how you run your own planet, but if you threaten to extend your violence, this Earth of yours will be reduced to a burned-out cinder. Your choice is simple: join us and live in peace, or pursue your present course and face obliteration. We shall be waiting for your answer. The decision rests with you.
At the end of the movie, Klaatu is shot and dies, then is brought back to life by the robot, Gort. Helen asks whether Gort has the power of life and death but Klaatu explains that the resurrection is not permanent and that the power of life and death is reserved to the Almighty Spirit. Originally Klaatu's death and resurrection were meant to be permanent, reinforcing his God-like powers. This did not make the Breen Office - the film industry's censors – happy, feeling that it was too left wing. At the insistence of the Breen office the director Robert Wise and writer Edmund H North put in the line, "That power is reserved for the Almighty Spirit". Both Wise and North hated the line and thought it completely inappropriate - negating the concept of Klaatu's race being all-knowing and all-powerful - but the studio wouldn't back them up and they were forced to put it in.