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 an undeclared war in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and South Vietnam. North Vietnam was supported by the Soviet Union, China, and other communist allies; South Vietnam 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 from some US perspectives. It lasted some 19 years with direct U.S. involvement ending in 1973 following the Paris Peace Accords, and included the Laotian Civil War and the Cambodian Civil War, resulting in all three countries becoming communist states in 1975.
American military advisors began arriving in what was then French Indochina in 1950 to support the French in the First Indochina War against the communist-led Viet Minh. Most of the funding for the French war effort was provided by the U.S. After the French quit Indochina in 1954, the US assumed financial and military responsibility for the South Vietnamese state. The Việt Cộng, also known as Front national de libération du Sud-Viêt Nam or NLF (the National Liberation Front), a South Vietnamese communist common front aided by the North, initiated a guerrilla war against the South Vietnamese government in 1959. U.S. involvement escalated in 1960, and continued in 1961 under President John F. Kennedy, with troop levels gradually surging under the MAAG program from just under a thousand in 1959 to 16,000 in 1963.
By 1964, there were 23,000 U.S. troops in Vietnam, but this escalated further following the 1964 Gulf of Tonkin incident, in which a U.S. destroyer was alleged to have clashed with North Vietnamese fast attack craft. In response, the Gulf of Tonkin Resolution gave President Lyndon B. Johnson broad authorization to increase U.S. military presence, deploying ground combat units for the first time and increasing troop levels to 184,000. Past this point, the People's Army of Vietnam (PAVN), also known as the North Vietnamese Army (NVA) engaged in more conventional warfare with US and South Vietnamese forces. Every year onward there was significant build-up of US forces despite little progress, with Robert McNamara, one of the principal architects of the war, beginning to express doubts of victory by the end of 1966. 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 U.S. conducted a large-scale strategic bombing campaign against North Vietnam. The Tet Offensive of 1968, proved to be the turning point of the war; despite years of American tutelage and aid the South Vietnamese forces were unable to withstand the communist offensive and the task fell to US forces instead. The Tet Offensive showed that the end of US involvement was not in sight, increasing domestic skepticism of the war. The unconventional and conventional capabilities of the Army of the Republic of Vietnam (ARVN) increased following a period of neglect and became modeled on heavy firepower-focused doctrines like US forces. Operations crossed international borders; bordering areas of Laos and Cambodia were used by North Vietnam as supply routes and were heavily bombed by U.S. forces.
Gradual withdrawal of U.S. ground forces began as part of "Vietnamization", which aimed to end American involvement in the war while transferring the task of fighting the communists to the South Vietnamese themselves and began the task of modernizing their armed forces. Direct U.S. military involvement ended on 15 August 1973 as a result of the Case–Church Amendment passed by the U.S. Congress. The capture of Saigon by the NVA in April 1975 marked the end of the war, and North and South Vietnam were reunified the following year. The war exacted a huge human cost in terms of fatalities (see Vietnam War casualties). Estimates of the number of Vietnamese soldiers and civilians killed vary from 966,000 to 3.8 million. Some 275,000–310,000 Cambodians, 20,000–62,000 Laotians, and 58,220 U.S. service members also died in the conflict, and a further 1,626 remain missing in action. The Sino-Soviet split re-emerged following the lull during the Vietnam War and confllict between North Vietnam and its Cambodian allies in the Royal Government of the National Union of Kampuchea, and the newly-formed Democratic Kampuchea begun almost immediately in a series of border raids by the Khmer Rouge and erupted into the Cambodian–Vietnamese War, with Chinese forces directly intervening in the Sino-Vietnamese War. The end of the war and resumption of the Third Indochina War would precipitate the Vietnamese boat people and the bigger Indochina refugee crisis, which saw an estimated 250,000 people perish at sea. Within the US the war gave rise to what was referred to as Vietnam Syndrome, a public aversion to American overseas military involvements, which together with Watergate contributed to the crisis of confidence that affected America throughout the 1970s.