The Day the Earth Stood Still (1951)

Revealing mistake: The man from Mars is in an elevator with a woman when the power goes out. He tells her he cut the electricity and gas powered engines all over the world for a half an hour. They proceed to show clips of the power out in different parts of the world. When they get to Big Ben in London, you can see a motorboat cruising down the river in the background. If the boat were powered by wood or coal, there would be plume of smoke trailing it. If it were powered by oil (ie. diesel) electricity is required to pump the fuel into the ejectors. (01:02:05)

Klaatu: Perhaps before deciding on a course of action, you'd want to know more about the people here - to orient yourself in a strange environment.
Mrs. Barley: There's nothing strange about Washington, Mr. Carpenter.
Klaatu: A person from another planet might disagree with you.
Mrs. Barley: If you want my opinion, he came from right here on Earth. And you know where I mean.
Mr. Krull: They woudn't come in spaceships, they'd come in airplanes.
Mrs. Barley: I woudn't be too sure about that.

Trivia: In order to make Gort's body appear seamless, two different costumes were created, one that opened in the back for front shots and one that opened in the front for rear shots. Unfortunately, as someone has already pointed out on this site, it was sometimes rather difficult to co-ordinate which suit was needed for which shot and thus there are several times in the film where Gort's seams can be seen.

Question: For such and advanced species, wouldn't Klatu's people know that "reducing Earth to a burnt out cinder" (for its warlike ways) would also punish those billions of innocents who have no say in the policies of their governments or military? Wouldn't it be more effective to police the earth and neutralize any weaponry that we shot into space?