As the current empire, the USA began 1900 with a strong, growing country, an economic powerhouse destined to lead the world, and best of all, their currency was all denominated in gold and silver.
The USA removed its link in gradual phases, but several major milestones stand out in history. The first of these was the creation of the Federal Reserve, in 1913, just before World War One, handing control of the US national currency to private banking interests. To emphasise the importance of this, remember Amschel Rothschild’s quote :-
“Give me control of a nation’s money, and I care not who makes the laws.”
― Amschel Rothschild
James Corbett’s superb “Century of Enslavement – History of the Federal Reserve” video and podcast documentary is highly recommended for the full history of how that happened.