Hmm...I doubt we would have a WWIII. Terrorism, little fights here and there, some tests here, some plots there, and probably a lot going on behind the scenes with intelligence agencies, but a full-scale world war?
I don't think anyone would survive that because even IF you managed to survive the immediate war, you'd never survive the after-effects, many of which may make themselves apparent years or decades afterwards.
Now, if you ask me which country will become the most powerful in the next few decades, that's something else, and I'm going to go with either Russia, China, or India in conjunction with alliances with Europe and the United States, where all major actors are relatively powerful but no one supremely.
And I think the power of the USA will decrease substantially within the next few decades, where it will be a world power equivalent to the EU at best and a solitary EU country at worst (Great Britain, France or Germany).