Idk, and hopefully won't have to ever... but there are a lot of things that can lead to this. A lot of the world hates us... and countries like North Korea that hate us AND have nukes are especially dangerous, our economic system = FAIL and political corruption is despicable.
Political influence, economic influence, the country itself... 'disbanding' or being taken over by another country?
Some of those, like political and economic are bound to fluctuate, but I think they can dip & return to where they used to be. I think for the U.S.A. to really fundamentally change and 'go away', I think it would because of it being replaced by some.. united earth government, if you will. I don't think something like that would happen any time soon though.
There could be several contributing factors to the US's downfall (In the likely case there is one), but no single definitive one just yet. I'm thinking it will most likely have something to do with the economic crisis or the war with the Middle East. Most likely, it could be an accumulation of all the factors eventually causing America to up and crash.
If the US overextends it's empire, it will fall. The Soviet Union overextended into E. Europe, Central Asia, and Afghanistan. They also spent way too much money on military and nuclear research while spending very little on the people. This caused rebellion. If the US continues its war crusade, it will inevitably fall.
The United States will fall if it doesn't adopt more socialist economic reforms as all of Scandinavia and most of Europe already has. All economic free market systems will fail or face unremitting recessions without radical democracy and socialist reform.
