mutually assured destruction is what holds everything together.
China nukes us, even just once, china is gone. But the problem with that is then other countries get pissed at us, and destroy us. It's all an extremely delicate situation. No nukes would be dropped in the middle east, because it's pointless. Think about it, we're going after terrorists who destroy their own cities, what good would destroying the whole thing for them do?
Nukes were dropped in Japan in part to try to prevent a massive invasion of the main islands that would have gotten millions killed. And it was a way to say "The allies won, now don't fuck with us again"