After World War 2, the US emerged as the dominant economic power due to its large gold reserves. In 1944, the Bretton Woods agreement established the US dollar as the currency to trade gold, backed by the US gold reserves. This created strong global demand for the US dollar. However, by 1971 rising US debt and spending on the Vietnam War depleted US gold reserves. In response, President Nixon ended dollar convertibility to gold, disrupting the global monetary system. To maintain demand for the dollar, the US negotiated an agreement where OPEC would sell oil using only the US dollar, establishing the "petrodollar" system and securing the dollar's status as the world's reserve currency.