One of the benefits of sovereignty is that you are incapable of going bankrupt. Should the US become insolvent (ie: the government no longer represents the will of the people and is abolished, or our member states all secede, or any other number of scenarios that END the SOVERIEGN state of the United States) our debts would then be settled by other countries seizing US assets (since the US is lawless and would no longer have the means to defend itself).
Basically: we've the most powerful military in the world. We're hegemon in the free market/civil rights/free trade/globalized corporate world order. We have the most might and therefore the HIGHEST sovereignty on earth. Other countries recognize that our might and wealth is rooted in our HUGE consumer marketplace. Our HUGE consumer marketplace is resultant of a vast middle class that's supported by our well-balanced system of governance.
So, our middle class would have to shrink to non-existence thanks to an inability to provide resources (impossible... the US was established at a point in history when no other sovereign state existed in the land surrounding, therefore our fledgling union was able to snatch as much territory as possible, eventually expanding to cover BOTH COASTS of A LARGE CONTINENT and EVERYTHING IN BETWEEN. History improved this advantage with the establishment of TWO stable states to the north and south. The US isn't UN-invadeable, but thanks to our modern satellite technology we're pretty close to impenetrable.), then our state would have to be unable to defend itself against enemies who WANT America destroyed (second amendment and patriotic ideals make that unlikely... factions that HATE America are countered by many who owe their decent lives to the promise of this free land), and finally states that want America's resources would have to come in and take control before the NATION (people) of America reestablishes the STATE (sovereign political entity) of America.
It's really difficult. The US rides a GREAT world-image outlook of being 100% democratic and very individual rights-oriented regardless of nationality. We're seen as a place with the MOST commecial friction and thusly the highest number of opportunities to personally succeed. We're perceived as an INCREDIBLY stable investment by most foreign states, because as a result of our dependence on a large market for our commercial success any investment in improving America and lessening restrictions on our market's spending-power eventually benefits the nations lending just the same.
China is taking advantage of this as their nation is industrializing. As OUR middle class recovers and continues buying goods from abroad (made in China), THEIR middle class will grow and China's infrastructure will improve to support it.
The only real end is that our populations outgrow our planet's ability to sustain them. It's kinda like the arms race leading to WWI.