Here is a list of the only ways the US at the moment can be defeated:
1929 style Stockmarket Crash: Not likely, but it is one way. No companies could afford to pay workers, build materials, we all know what happened then. Problem is, every other nation in a postion to attack the US succsessfully would be in the same boat. So yeah, that is very unlikely.
A Second Civil War: About as likely as the Stockmarket crash. The US, as its name implies is UNITED. There may be hostility between politcal ideals (Republicans and Democrats) but not enough tension for the states to proclaim indepenece and begin fighting each other.
The world at the last minute Ganging up on America: You don't need me to tell you how unrealistic that is.
The Alien idea I keep reading on here: Good to see people are using their imaginations, but It don't look likey at all. It is definatly a way to defeat the US, but it ain't gonna happen.
A Extremist Group hold Government: An outlandish conspiracy I heard once, eg, The Nazi's or Communists or other groups like that holding office. This in the scale of unlikelyness is right before the Aliens coming down here. An extremist group would never hold office in the US Lawfully.
Insurgency by Terrorists: It would cause problems, break down of law and order in parts, rampant terror attacks would do a considerable amount to hurt the US, but with help from allies, and Patriotic Militia's, as well as the entire US Defence Force, it would not defeat the US.
The US Military can be defeated in battle, but they will have enough allies to help protect them to rebuild their forces to unleash hell upon whoever defeated them. The biggest deterrent to attacking the US, is the fact the could overthrow almost every national government by itself. With the help from their allies, every nation in the world, and still have resoruces to rebuild the nation.
The US CAN be defeated like all nations, but provided its leaders learn from history, and pay attention to world events, it is VERY UNLIKELY that it will happen