Perhapse the trillions of dollars of debt combined with the increasing amount of outsourcing north american industry. Add more wars than a nation can handle, and the empire may crumble, especially if somebody powerfull (china perhapse) sees a weakened nation and decides to invade. But realistically I think America will just eventually lose their status as a superpower. After all, the Egyptian empire crumbled a long long time ago, but Egypt didn't just disapear off the map.