If it was the entire world and strictly conventional warfare - no nukes, no cyberwar, not attacked by subversive groups in the US, etc. Properly co-ordinated, the world would defeat the US, in combat. read more
No country would attack the US on its own. Using North Korea or Iran would be another matter. Then Russia and China might have other plans. Or if Al Quida succeeded in an attack which actually caused choas through much of our nation, Russia wouldn't allow us to fall to such enemies without taking a shot. read more