The Vietnam War, a protracted and controversial conflict that lasted from 1955 to 1975, remains one of the most significant events in the history of the 20th century. The war, fueled by Cold War tensions and ideological differences, pitted the communist North Vietnam, supported by its communist allies, against the government of South Vietnam, backed primarily by the United States. The U.S., driven by the fear of the Domino Theory - the belief that if one state in a region came under the influence of communism, then the surrounding countries would follow - committed vast resources and manpower in an attempt to prevent a communist takeover of South Vietnam.