Well, it doesn't necessarily have to be a world war with everyone choosing up between two sides.

What I mean is there is already plenty of fighting going on that is not related:
US vs. Afghanistan,
Russia vs. Chechneya,
Israel vs. Palestine,
crappy African nation X vs. crappy African nation Y (there's almost always a couple of crappy African nations fighting)

There may be some confilcts that are quite unrelated (like India vs. Pakistan) erupting throughout the world. It looks like Venezuela is getting ready for a civil war.