An oversimplified but useful model is that, internationally, any country's money is, in the eyes of another country, just a promise to repay. If one country sen

An oversimplified but useful model is that, internationally, any country's money is, in the eyes of another country, just a promise to repay. If one country sends goods to another and receives money for it, the exporting country has effectively received a bunch of promises that the importing country will eventually (repatriate the money and) give them stuff in return. It's more complicated because the exporting country could instead give the (say) dollars to a third country in return for real stuff. But still what gives those dollars is the reasonable expectation that others will continue to value them enough to give you actual useful stuff for them.