I was flipping through the dictionary the other day looking for another word and happened to pause for a moment on the entry for "nucleus". It turns out nucleus is from the Latin for kernal. More interesting, though, is that it's a diminutive form of the Latin for nut. Therefore, the original meaning of nucleus is "a small nut".
So, as far as I can tell, a nuclear war is a lot of trouble started by one little nut. :-p