Barnhardt: One thing, Mr. Klaatu: suppose this group should reject your proposals. What is the alternative?
Klaatu: I'm afraid there is no alternative. In such a case, the planet Earth would have to be... eliminated
Barnhardt: Such power exists?
Klaatu: I assure you, such power exists.
From The Day the Earth Stood Still (1951) (Quote courtesy of Internet Movie Database)