The US owes China about a trillion dollars.
Most of this is in long term bonds, so there is a specific period when it comes due. If China stopped buying the bonds, it’d still be 30 years before we had to pay off every last dollar.
So we would be faced with a number of options, beginning with simply selling new debt to someone else. The government doesn’t have too much trouble finding buyers for it’s debt, worse case, it pays a bit higher interest rate. And if they wanted to, the government could simply print the trillion dollars. That would be inflationary, of course, but new money has been coming on

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