I like Klaatu's farewell speech...

I am leaving soon and you'll 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 signs 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.