God,you're extremely fucking stupid!" The Untied States would win. The US has more and better allies than any other country, and it has a far more advanced military. Sure one obvious answer is China, because everyone wants to think China is this unbeatable superpower. But China has no real way to mobilize its troops, sure it would be able to defend its country but it would not be able to efficiently invade a country that is separated by water. And most Chinese troops wouldn't even make it to the mainland US, the American Navy is way to superior. And I doubt nukes would be used. The only people dumb enough to use them are countries like Iran, and Korea. And they won't have enough to "end the world". The main reason the US has nukes is to deter countries like that from even thinking about using them. "