The "correct" answer can be whatever you think: I've heard the military referenced many times. I've heard oil (petro dollar), although the Bretton woods accords are falling apart now. My own opinion is federal reserve notes are simply accepted everywhere and they are ubiquitous. Other countries accept federal reserve notes because their own currencies are weak. All of this is rapidly coming to an end. The well connected are trading their fiat for hard commodities like gold, silver and real estate. They want the currency to collapse because they are in a position to become much wealthier as us deplorables get poor from being in debt. Once fiat collapses, these people will take our property.