Consider these two facts:
1) There were two 'world' wars in the previous century.
2) The 'world' has a good chance of existing for another 45 million centuries.
Based on those figures, I'd say that a third world war is pretty much inevitable. What I can't predict is when it will happen and who - or what - will fight it.

A lot can happen in 4,500 million years. The protagonists might be a different species altogether. They might be intelligent machines. Or they might be fungi and bacteria squabbling over the decaying mass of human remains after we've stripped the planet bare of all plant life.