I believe we already had WWIII, that would have been the Cold War. It took place between 2 major sides, Vietnam, Korea, Cuba, etc, were all battle grounds for it. And it was fought all over the world. Now, mose people wouldn't say it was a world war because it didn't have unified Front. But, you are never going to see a unified front in war anymore. It's never going to be Trench Warfare or Boundaries like WWI and WWII. It's going to be hot spot and regions, all over an Ideology.
I will be called crazy or stupid or whatever by all the Left-wing Yah-hoos on this forum, but I think we are in WWIV, the war against Radical Fundilmentalism. We got hit in many places before 9/11, but 9/11 was our Pearl Harbor. We didn't fight for French Indendance until After Pearl Harbor, and we made a push to fight all the way to Germany. It's funny the Parallels: we got hit in Pearl Harbor, by Imperial Japanesse, but we fought a front in Europe, but Nazi's didn't attack us, why did we go there? Just like we weren't attacked in 9/11 by Iraq, but why are we there?