You could probably read through the speeches of some historical dictators to find a good way to express the idea of "the real politics is what things fall apart in to when order breaks down." Sure, the national debt exists on one level higher than the stuff the defense spending is buying, but so is our trade with other countries and most of our livelihoods. If the civility of our relationship with, say, China breaks down to the point that it's obvious we won't pay our debts, a lot of other things will break down along with it - perhaps enough to qualify as a gigantic disaster.

The US will be able to keep foreign soldiers off its territory but you and I could be out on the streets with a whole lot less.