In 1971 Saudi-Arabia asked the US Military to protect them, so they made a deal with the US. In agreement, the US Military asked that Saudi-Arabia sell all or their oil globally - dollar denominated. This gave much strength to the Dollar. Sadly, Saudi Arabia changed this agreement on August 24th, 2021. Now, their oil can be sold by something other than the Dollar - and that it the beginning of the crash for the US Dollar.