During the Cold War, the Soviet Union and Eastern Bloc countries held US dollars but didn't want to deposit them in US banks (fear of freezing). They deposited them in European banks instead, creating the first "Euro Dollars."
European banks started lending these dollars to other customers, creating a market for dollar-denominated loans outside the US. This market grew rapidly as it offered more flexibility than US banking regulations.
The Euro Dollar market expanded globally. Banks in London, Tokyo, Singapore, and other financial centers began holding and trading US dollars. The market became a major source of global liquidity.
The Euro Dollar market is now estimated to be in the trillions of dollars. It's a critical part of global finance, providing liquidity for international trade and cross-border transactions.