The most immediate and foreseeable threat to the U.S., and the broader global system, is economic collapse:
All that has to occur is for the existing projections of unsustainable debt to continue. Or for the additional shock of another costly crisis, such as a massive earthquake (as occurred in Japan). Or something like 9-11-2001 that took 1 trillion dollars out of the economy. Or a doubling of interest rates on the U.S. national debt. Or a massive war. Or an economic collapse in Europe or China, that sends shockwaves through the already fragile U.S. system. Or vice versa, as occurred in Germany and the rest of Europe after the 1929 collapse.
Until we have a president who starts running annual surpluses, I won't be optimistic that there's any way we aren't doomed. And from what I've seen, Europe, China, Japan, and South America are just as much in debt, and danger.
