While nuclear war IS one of the most likely options, I don't think it'll be a case of world war. My friend explained it to me recently during a game of Chrononauts (a card game where you get to mess around with time like assassinating Hitler, making the Titanic hit a derelict underwater mine and exploding rather than hitting an iceberg... even creating World War 3...)
the way he explained it was that the only reason that WW3 would happen would be if we hadn't had the second World War and seen the destructive power of a Nuke in action. Because of that one event in history, we've learned they're not just toys to be played with and world leaders are a lot more careful because of it. While there are rogues and terrorists who would threaten to use a nuke, and possibly actually go through with it, it becomes in everyone's best interest, friend and foe alike, to stop such a tragedy from occurring. Because we've learned NOT to use them, there's much less likely a chance that we'll go into a nuclear war as opposed to if Hiroshima never happened and we didn't realise the potential of such a weapon until everyone had one ready to launch.