The Vietnam War (Vietnamese: Chiến tranh Việt Nam), also known as the Second Indochina War, and in Vietnam as the Resistance War Against America (Vietnamese: Kháng chiến chống Mỹ) or simply the American War, was a conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975, with U.S. involvement ending in 1973. It was the second of the Indochina Wars and was officially fought between North Vietnam and the government of South Vietnam. The North Vietnamese army was supported by the Soviet Union, China, and other communist allies; the South Vietnamese army was supported by the United States, South Korea, the Philippines, Australia, Thailand and other anti-communist allies. The war is considered a Cold War-era proxy war by some US perspectives. The war would last approximately 19 years and would also form the Laotian Civil War as well as the Cambodian Civil War, which resulted in all 3 countries becoming communist states in 1975.

There are several competing views on the conflict. Some on the North Vietnamese and National Liberation Front side view the struggle against U.S. forces as a colonial war and a continuation of the First Indochina War against forces from France and later on the United States, especially in light of the failed 1954 Geneva Conference calls for elections. Other interpretations of the North Vietnamese side include viewing it as a civil war, especially in the early and later phases following the U.S. interlude between 1965 and 1970, as well as a war of liberation. In the perspective of some, the Provisional Revolutionary Government of the Republic of South Vietnam, the successor to the Việt Cộng, was motivated in part by significant social changes in the post-World War II Vietnam, and had initially seen it as a revolutionary war supported by Hanoi. The pro-government side in South Vietnam viewed it as a civil war, a defensive war against communism, or were motivated to fight to defend their homes and families. The U.S. government viewed its involvement in the war as a way to prevent a communist takeover of South Vietnam. This was part of the domino theory of a wider containment policy, with the stated aim of stopping the spread of communism.