This may be news to you, but debt and exchange still take place internationally, have taken place internationally since long before there were superpowers, and also take place in zones where no law is enforced.
As for the word of the United States, I'm guessing you've never been to an Indian reservation.
It may be in poor taste to say it, but if the Native American nations had bought American Dollars rather than trading for goods with "inherent" value, history may have played out very differently.
It's not poor taste; it just shows you don't know much about history.
That only really goes for people on American soil. Of course, most governments that owe money to the US are in such deep shit that they couldn't pay the cleaning lady next week without a new loan this week. New loans tend to be problematic if you don't pay interest on your current loan.
To say that the word of the United States is worthless is to ignore reality.