Why didn't the United States declare all 35 trillion foreign debts null and void?

Think about it, after China fell out with the Soviet Union, why did it have to pay back the money it owed to the Soviet Union, and pay it back in advance with guaranteed quality and quantity.

The Soviet Union meant that if there were difficulties, it could be paid on time, or even postponed.

Money is like flowing water, it will come back after it is spent.

Reputation is like youth, it will never come back once it is gone.