Maybe, but not by itself. Iran is a nobody on the world scene. People would scream and complain if the US invaded tomorrow, and the war would take even more troops away from other posts, but I really doubt it would get too much worse than it already is with Iraq. What Iran needs to start WW3 are some allies, especially if they're big, like, oh I dunno, Russia. Europe is also close enough to have a significant interest in what goes on in the region, but they're mostly interested in sorting out the messes in Africa.\n\nIf Russia or even a significant number of other Middle-Eastern or central Asian countries posted some of their military in Iran to physically protect it, they would all be drawn into an American invasion and the war would quickly spread. Also, the region is dangerous enough as is; Iran borders Pakistan, who like it's antagonistic neighbor India has nuclear weapons, and Isreal is also suspected of such. A major war enveloping the Middle East would likely touch off these sideshows too. Maybe China would take the opportunity to go for Taiwan while the US is too busy to look, and maybe North Korea will try moving south again. Who knows how far it could spread?