The Vietnam War was a conflict between North Vietnam, supported by its communist allies, and South Vietnam and its principal ally, the United States. The war began in 1955 and ended in 1975 with a communist victory that reunited Vietnam under communist rule. The United States intervened in the war to stop the spread of communism in Southeast Asia based on the Domino Theory that argued if one country fell to communism then neighboring countries would follow. Key events included the Battle of Dien Bien Phu which led France to withdraw from Vietnam, the Gulf of Tonkin Incident which prompted greater US involvement, and the fall of Saigon in 1975 which marked the end of the war.