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 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, considered a Cold War-era proxy war by some, lasted 19 years, with direct U.S. involvement ending in 1973, and included the Laotian Civil War and the Cambodian Civil War, resulting in all three countries becoming communist in 1975.
The conflict emerged from 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 support 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 common front under the direction of North Vietnam, initiated a guerrilla war in the south. North Vietnam had also entered Laos in the mid-1950s in support of insurgents, setting up the Ho Chi Minh trail to supply and reinforce the Việt Cộng and increased in 1960. U.S. involvement escalated under President John F. Kennedy through the MAAG program from just under a thousand in 1959 to 16,000 in 1963. By 1963, the North Vietnamese had sent 40,000 soldiers to fight in South Vietnam. North Vietnam was heavily backed by the People's Republic of China, which in addition to supplying arms as the USSR did, also sent hundreds of thousands of PLA servicemen to North Vietnam to serve in support roles.
By 1964, there were 23,000 US advisors in South Vietnam during the 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 (known also as the 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, expressing doubts of victory by the end of 1966. U.S. and South Vietnam forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery, and airstrikes. The U.S. also conducted a large-scale strategic bombing campaign against North Vietnam and Laos.
The Tet Offensive of 1968 showed the lack of progress with these doctrines as the NLF and PAVN mounted large-scale urban offensives throughout 1968, turning US domestic support against the war. The Army of the Republic of Vietnam (ARVN) expanded following a period of neglect after Tet, modeled on US doctrine. The NLF sustained heavy losses during the Tet Offensive, losing over half its strength in a matter of months, which combined with subsequent U.S.-ARVN operations in the rest of 1968, nearly wiped out the southern insurgency. The CIA's Phoenix Program further degraded the NLF's membership and capabilities. By the end of the year, the NLF insurgents held almost no territory in South Vietnam, and their recruitment dropped by over 80% in 1969, signifying a drastic reduction in guerrilla operations and necessitating increased use of NVA regulars from the north. In 1969, North Vietnam declared a Provisional Revolutionary Government in South Vietnam in an attempt to give the reduced NLF a more international stature, but the southern guerrillas from then on were sidelined as PAVN forces begun more conventional Combined arms warfare. Operations crossed national borders: Laos was invaded by North Vietnam early on while Cambodia was used by North Vietnam as a supply route starting in 1967; the route through Cambodia began to be bombed by the U.S. in 1969, while the Laos route had been heavily bombed since 1964. The deposition of the monarch Norodom Sihanouk by the Cambodian National Assembly resulted in a PAVN invasion of the country at the request of the Khmer Rouge, escalating the Cambodian Civil War and resulting in a U.S.-RVN counterinvasion.
After 1968, Nixon's policy of "Vietnamization", saw the conflict fought by an expanded ARVN with U.S. forces sidelined and increasingly demoralized by domestic opposition and reduced recruitment. U.S. ground forces withdrew by late 1971, and U.S. involvement became limited to air support, artillery support, advisers, and materiel shipments. The ARVN, buttressed by said U.S. support, stopped the largest and first mechanized PAVN offensive to date during the Easter Offensive of 1972, resulting in mutually heavy casualties, but failed to recapture all territory, leaving its military situation difficult. The Paris Peace Accords saw all US forces withdrawn and intervention prohibited by the US Congress on 15 August 1973 as a result of the Case–Church Amendment. The Peace Accords were broken almost immediately, and fighting continued for four years following the withdrawal of U.S. ground forces and two years following the withdrawal of the remaining U.S. forces, though U.S. material support continued at a much reduced rate. The three-year period of 1972 to 1974 saw heavy fighting and constituted the war's bloodiest years for the ARVN. The 1975 Spring Offensive culminated in the capture of Saigon by the PAVN in April 1975; this 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 conflict 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 that erupted into the Cambodian–Vietnamese War, with Chinese forces directly invading in the Sino-Vietnamese War and subsequent border conflicts. Insurgencies were fought by the unified Vietnam against insurgencies in all three countries. 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 millions of refugees leave Indochina (mainly southern Vietnam), an estimated 250,000 of whom perished 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.