The Vietnam War occurred in Vietnam, Laos, and Cambodia from 1959 to 1975 between communist North Vietnam and the communist Viet Cong against South Vietnam and the United States. The US entered the war to prevent the spread of communism in Southeast Asia according to the Domino Theory. Several US presidents escalated US involvement, but Nixon began withdrawing troops and negotiated a ceasefire, effectively ending US involvement despite later attempts to support South Vietnam.