For most of the 20th century, the United States exercised sole dominance in the Gulf. Its armed forces kept other external powers at bay, and punished local governments for actions that posed a threat to regional stability. Most important were Washington’s shifting relations with Saudi Arabia, Iran and Iraq. By the early 21st century, US predominance had diminished, opening opportunities for Russia and China. Both Moscow and Beijing expanded joint military operations, arms shipments and economic connections with governments in the region, which portend more competitive interventions in future.