Basically the US, as a country, is like a rich person that spent an amazing shit ton of money on the best imaginable security for his house, ringing it with multiple moats, missile launchers, thousands of full-time heavily armed guards with the best possible weaponry and equipment, and on and on and on. In fact, this person went so overboard that he borrowed from anyone who would lend to him for all this crazy security. And while the multi-decade project of building this security apparatus was going on, he neglected to pay any attention to the source of his wealth. One by one, the companies he held stock in dissolved, the buildings he owned fell empty because of neglect, the partnerships and social contacts that ensured he stayed on top of the game fell away. His industries were eclipsed by newer, nimbler players. And by the time his security was perfect, he knew nothing but fear and hadn't yet realized that he no longer had anything worth protecting.
We're currently at the point where the creditors can still be kept from collecting by the massive military machine we've built, but eventually we'll run out of money to pay them, too. And then the whole house of cards comes down.