"Under an OPEC agreement, all oil has been traded in US dollars since 1971 (after the dropping of the gold standard) which makes the US dollar the de facto major international trading currency. If other nations have to hoard dollars to buy oil, then they want to use that hoard for other trading too. This fact gives America a huge trading advantage and helps make it the dominant economy in the world. As an economic bloc, the European Union is the only challenger to the USA's economic position, and it created the euro to challenge the dollar in international markets." (Quoted from article)