A 1970s handshake deal tied the world's oil trade to the US dollar

After the US abandoned the gold standard in 1971, Washington struck an arrangement with Saudi Arabia to price its oil exports exclusively in dollars in exchange for military protection, and the rest of OPEC followed suit. The deal, never one single signed treaty but a web of agreements through the 1970s, meant nearly every oil-importing country needed dollars, cementing the currency's global dominance for decades.

— US State Department / OPEC member states, Petrodollar recycling — 1970s