The practical framing would be as two follow-up questions: what do the lenders care about? And what happens empirically when debt spirals?

If lenders do nothing, then nothing really matters - keep borrowing and let your debt grow exponentially.

In practice though, lenders, in their wisdom or folly, get spooked when debt goes up. In the Greek debt crisis, it all started with debt in the region of 130% of GDP. Rolling the debt with more debt spiralled away as lenders wanted an ever higher interest.

So US would either need to start really inflating its debt and test the investors patience, print the cash and let inflation run away, or tax and cut spending.

In the first scenario it kind of doesn't matter where the limit is - people sometimes argue about magic levels. The issue is that eventually the debt grows exponentially, so once it's out of control, it will exceed any reasonable level pretty quickly.

Can the US convince the world that their debt is special? I'm not sure. My reading is that investors are already twitchy about US debt, for other reasons for now, but higher debt levels surely won't calm them down.

So really I think the US has no better choice than to keep its debt down.

> If lenders do nothing, then nothing really matters - keep borrowing and let your debt grow exponentially.

Lenders are currently doing nothing: that does not necessarily mean they will do nothing forever.

Because should the lender actually do something eventually, the lendee may be in a world of hurt. The borrower probably does not get to the point when lenders start doing something.

Everything I've ever seen shows that problems in these areas happen very slowly, and then all at once.

Previous similar (but totally different) situations ended in wars.