*cracks knuckles* All right. You get world peace. The nations of the world dismantle their war machines. Everything's going ok... until the aliens long predicted by science fiction arrive (it took them a while since this isn't science fantasy, and thus they couldn't travel faster than light). Aliens attack: everybody dies.
I wish for a monkey... not a nuclear monkey, not even a monkey in my general vicinity, I simply wish that a monkey would now exist which did not before... preferrably it can come into being however you choose in South America.
There. Ruin that!