On 15th August 1971 the USA defaulted on the Bretton Woods agreement. In their very good book ‘Money, Markets and Sovereignty’ Steil and Hinds point out that it is only since 1971 that the world has not based its currency on metal, as it had done for 4,000 years or so before.
https://www.jstor.org/stable/j.ctt1np9xt
I wonder what the next US default will look like?