You know how the aliens always say "We come in peace"? Well, what happens if we're all allergic to aliens? Then after a few days of tea and crumpets with Sergeant Zormac, the President starts getting hives. After a friendly game of tiddlywinks with Mr. Blimkor, the Queen develops an oddly colored rash. Soon enough all the world leaders start getting sick, and we're stuck with a leaderless world.
Then what happens? Naturally, Sergeant Zormac becomes Emperor Zormac, Mr. Blimkor becomes Mrs. Blimkor, and we have been duly conquered. "We come in peace"? Doesn't look like it anymore, does it?
So the moral of the story is next time anyone tells you that they come in peace, blast the suckers before they go all Cortez on you.