Can be either South Korea or Japan, I doubt they will target America so brazenly.
But if they nuked anywhere, what exactly would happen afterward? I mean everyone always claims we would just nuke them in response, but then wouldn't that provoke them into launching another one themselves and causing horrible damage? Negotations are out of the picture and, if we send our military there, they might just nuke themselves in a taking you with me scenario.
I predict China and Russia will just reveal their allegiances and side with NK. Then there will be no nukes under the threat of mutually assured destruction and their alliance takes over the world. This is why I find it silly we allow China to protect North Korea as it is, it's only a matter of time.