Prior to 1971, the USD was backed by gold under the gold standard
After 1971, the USA went bankrupt thanks to the Vietnam War so we decided to back the dollar by something else, particularly oil, with a deal we made with the Saudi Monarchy
Everyone who tries to sell their dollars in non-petrodollars gets invaded
Saddam Hussien tried to sell Iraq oil for Euros, he got overthrown
Muammar Gaddafi tried to create a gold-backed African currency, he got overthrown