Yeah, I don't see the US getting invaded... citizens are pretty resilient. Not like people would sit still if tanks were rolling down their streets. Plus you have the whole logistic problem of how something like China or Russia would GET their troops/equipment to our soil.
I don't think the US would break up either though... but I think it certainly would be interesting to see what happens when the government is like, "Well, we don't have any money... fend for yourselves." In all honesty, I think the US kind of needs a painful thing like that anyway. So many citizens just expect the government to take care of them with all their entitlements and people don't really strive to do anything great anymore.
It's going to be crazy if the government gets to a point where they can't borrow money anymore... "Well, no one gets any retirement, social security or medical anymore... sorry all you guys who need it, I guess you're going to be living under a bridge."
Back to sort of the topic... no one wants to vote in any politicians who would be willing to fix the budget issues because that means people will stop getting things like social security. It's not sustainable to be borrowing hundreds of billions of dollars every year to pay the interest on the money they previously borrowed. Hopefully politicians will be able to turn it around, but not really sure how at this point... even if they did it, you would have riots and uprisings from citizens who no longer get their entitlements. If it ever comes to that point, I'll watch it from TV in another country.