The United States has become the world's largest oil exporter, upending a decades-old order long dominated by Saudi Arabia and Russia, a shift that tightens American companies' grip on energy markets as Washington's war with Iran reshapes global energy trade. America's ascendancy to the top spot marks a stunning reversal for a country that was dependent on Middle Eastern oil for decades and suffered from an oil embargo imposed by some OPEC members in 1973 to retaliate against U.S. support for Israel.