For decades, the U.S government has been spending much more than it takes in from taxes. To make up for the shortfall, it uses debt in the form of U.S. treasury bonds. They currently pay about 5% a year.

They have long been considered risk free. Boring, safe, low return investments. Companies or people who need absolutely reliability in their investments buy them - the elderly, pensions, insurance companies, banks the world over.

The U.S. has issued so many of these bonds that the total amount outstanding right now is $40T. This amount is so staggering that to simply pay that 5% in interest payments costs us more than it takes to fund our very large, expensive military.

If they don't pay it back, and declare all those bonds worthless - well all of those people who were relying on what they thought was a rock solid, safe investment go bust. Banks fail worldwide, pensions run dry, retirement funds suddenly are empty, all kinds of businesses collapse. It would make the financial crisis of 08 look like a joke, and it would be a true catastrophe.

That is almost surely not going to happen.

What could happen is that we enter a debt spiral - investors get worried we won't be able to pay it back, and view bonds as less than perfectly safe. They now want 6%. The U.S. has to pay even more in interest every year, so they issue more debt to roll it over, which makes it worse and we get to 7%, etc.

Typically in this situation, a country either quickly gets its act together and commits to reducing spending and raises taxes, or they just turn on the money printers, and use inflation to make that debt smaller in real terms. I have little faith in the U.S. to commit to fiscal austerity and expect them to try to inflate the debt away.