The Vietnam War was a Cold War military conflict that occurred in Vietnam, Laos, and Cambodia from November 1, 1955 , to April 30, 1975 when Saigon fell. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of South Vietnam, supported by the United States and other capitalist nations.

The Viet Cong, a lightly-armed South Vietnamese communist-controlled common front, largely fought a guerrilla war against anti-communist forces in the region. The North Vietnamese Army engaged in a more conventional war, at times committing large units into battle. U.S. and South Vietnamese forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery and airstrikes.

The United States government viewed involvement in the war as a way to prevent a communist takeover of South Vietnam and part of their wider strategy of containment. The North Vietnamese government viewed the war as a colonial war, fought initially against France, backed by the United States, and later against South Vietnam, which it regarded as a US puppet state. United States military advisors arrived beginning in 1950. U.S. involvement escalated in the early 1960s, with U.S. troop levels tripling in 1961 and tripling again in 1962. U.S. combat units were deployed beginning in 1965. Operations spanned borders, with Laos and Cambodia heavily bombed. Involvement peaked in 1968 at the time of the Tet Offensive. After this, U.S. ground forces were withdrawn as part of a policy called Vietnamization. Despite the Paris Peace Accords, signed by all parties in January 1973, fighting continued.

The Case-Church Amendment passed by the U.S. Congress prohibited use of American military after August 15, 1973 unless the president secured congressional approval in advance. The capture of Saigon by the North Vietnamese army in April 1975 marked the end of the Vietnam War. North and South Vietnam were reunified the following year.

The war exacted a huge human cost in terms of fatalities (See: Vietnam War casualties), including 3 to 4 million Vietnamese from both sides, between one point five to two million Laotians and Cambodians, and 58,159 U.S. soldiers.