Vietnamization was a policy of the Richard Nixon administration to end U.S. involvement in the Vietnam War through a program to "expand, equip, and train South Vietnamese forces and assign to them an ever-increasing combat role, at the same time steadily reducing the number of U.S. combat troops". Brought on by the Viet Cong's Tet Offensive, the policy referred to U.S. combat troops specifically in the ground combat role, but did not reject combat by the U.S. Air Force, as well as the support to South Vietnam, consistent with the policies of U.S. foreign military assistance organizations. U.S. citizens' mistrust of their government that had begun after the offensive worsened with the release of news about U.S. soldiers massacring civilians at My Lai (1968), the invasion of Cambodia (1970), and the leaking of the Pentagon Papers (1971).

At a January 28, 1969, meeting of the National Security Council, General Andrew Goodpaster, deputy to General Creighton Abrams and commander of the Military Assistance Command, Vietnam, stated that the Army of the Republic of Vietnam (ARVN) had been steadily improving, and the point at which the war could be "de-Americanized" was close. Secretary of Defense Melvin Laird agreed with the point, but not with the language: "What we need is a term like 'Vietnamizing' to put the emphasis on the right issues." Nixon immediately liked Laird's word.Vietnamization fit into the broader dtente policy of the Nixon administration, in which the United States no longer regarded its fundamental strategy as the containment of communism but as a cooperative world order, in which Nixon and his chief adviser Henry Kissinger were focused on the broader constellation of forces and the bigger world powers. Nixon had ordered Kissinger to negotiate diplomatic policies with Soviet statesman Anatoly Dobrynin. Nixon also opened high-level contact with China. U.S. relations with the Soviet Union and China were of higher priority than South Vietnam.

Nixon said Vietnamization had two components. The first was "strengthening the armed force of the South Vietnamese in numbers, equipment, leadership and combat skills", while the second was "the extension of the pacification program [i.e. military aid to civilians] in South Vietnam." To achieve the first goal, U.S. helicopters would fly in support; however, helicopter operations were too much part of ground operations to involve U.S. personnel. Thus, ARVN candidates were enrolled in U.S. helicopter schools to take over the operations. As observed by Lieutenant General Dave Palmer, to qualify an ARVN candidate for U.S. helicopter school, he first needed to learn English; this, in addition to the months-long training and practice in the field, made adding new capabilities to the ARVN take at least two years. Palmer did not disagree that the first component, given time and resources, was achievable. However: "Pacification, the second component, presented the real challenge...it was benevolent government action in areas where the government should always have been benevolently active...doing both was necessary if Vietnamization were to work."

The policy of Vietnamization, despite its successful execution, was ultimately a failure as the improved ARVN forces and the reduced American and allied component were unable to prevent the fall of Saigon and the subsequent merger of the north and south, to form the Socialist Republic of Vietnam.

The Vietnam War (Vietnamese: Chiến tranh Việt Nam), also known as the Second Indochina 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 and other anti-communist allies. The war is widely considered to be a Cold War-era proxy war. It lasted almost 20 years, with direct U.S. involvement ending in 1973. The conflict also spilled over into neighboring states, exacerbating the Laotian Civil War and the Cambodian Civil War, which ended with all three countries becoming communist states by 1975.

The conflict emerged from the First Indochina War between the French colonial government and a left-wing revolutionary movement, the Viet Minh. After the French military withdrawal from Indochina in 1954, the U.S. assumed financial and military support for the South Vietnamese state. The Việt Cộng (VC), a South Vietnamese common front under the direction of North Vietnam, initiated a guerrilla war in the south. North Vietnam had also invaded Laos in 1958 in support of insurgents, establishing the Ho Chi Minh Trail to supply and reinforce the Việt Cộng.: 16  By 1963, the North Vietnamese had sent 40,000 soldiers to fight in the south.: 16  U.S. involvement escalated under President John F. Kennedy through the MAAG program, from just under a thousand military advisors in 1959 to 23,000 in 1964.: 131 In the Gulf of Tonkin incident in August 1964, a U.S. destroyer clashed with North Vietnamese fast attack craft. In response, the U.S. Congress passed the Gulf of Tonkin Resolution and gave President Lyndon B. Johnson broad authority to increase U.S. military presence in Vietnam. Johnson ordered the deployment of combat units for the first time and increased troop levels to 184,000. The People's Army of Vietnam (PAVN) (also known as the North Vietnamese Army or NVA) engaged in more conventional warfare with U.S. and South Vietnamese forces (Army of the Republic of Vietnam (ARVN)). Despite little progress, the U.S. continued a significant build-up of forces. 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.: 371–4 The communist Tet Offensive throughout 1968 caused U.S. domestic support for the war to fade. The VC sustained heavy losses during the Offensive and subsequent U.S.-ARVN operations.: 481  The CIA's Phoenix Program further degraded the VC's membership and capabilities. By the end of the year, the VC insurgents held almost no territory in South Vietnam, and their recruitment dropped by over 80%, signifying a drastic reduction in guerrilla operations, necessitating increased use of PAVN regular soldiers from the north. In 1969, North Vietnam declared a Provisional Revolutionary Government (the PRG) in the south to give the reduced VC a more international stature, but from then on, they were sidelined as PAVN forces began more conventional combined arms warfare. By 1970, over 70% of communist troops in the south were northerners, and southern-dominated VC units no longer existed. Operations crossed national borders: North Vietnam used Laos as a supply route early on, while Cambodia was also used starting in 1967; the U.S. bombed the Laotian route starting in 1964, and the Cambodian route in 1969. The deposing 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.-ARVN counter-invasion.

In 1969, following the election of U.S. President Richard Nixon, a policy of "Vietnamization" began, which 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 had largely withdrawn by early 1972 and support was limited to air support, artillery support, advisers, and materiel shipments. The ARVN, with U.S. support, stopped the first and largest mechanized PAVN offensive during the Easter Offensive of 1972. The offensive failed to subdue South Vietnam, but the ARVN itself failed to recapture all lost territory, leaving its military situation difficult. The Paris Peace Accords of January 1973 saw all U.S. forces withdrawn; the Case–Church Amendment, passed by the U.S. Congress on 15 August 1973, officially ended direct U.S. military involvement.: 457  The Peace Accords were broken almost immediately, and fighting continued for two more years. Phnom Penh fell to the Khmer Rouge on 17 April 1975, while the 1975 Spring Offensive saw the Fall of Saigon by the PAVN on 30 April; this marked the end of the war, and North and South Vietnam were reunified the following year.

By 1970, the ARVN was the world's fourth largest army, and the PAVN was not far behind with approximately one million regular soldiers.: 770  The war exacted an enormous human cost: estimates of the number of Vietnamese soldiers and civilians killed range from 966,000 to 3 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. Conflict between North Vietnam and its Cambodian allies in the Royal Government of the National Union of Kampuchea, and the newly formed Democratic Kampuchea began almost immediately in a series of border raids by the Khmer Rouge, eventually escalating into the Cambodian–Vietnamese War. Chinese forces directly invaded Vietnam in the Sino-Vietnamese War, with subsequent border conflicts lasting until 1991. The unified Vietnam fought 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 larger Indochina refugee crisis, which saw millions of refugees leave Indochina (mainly southern Vietnam), an estimated 250,000 of whom perished at sea. Within the U.S., the war gave rise to what was referred to as Vietnam Syndrome, a public aversion to American overseas military involvements, which together with the Watergate scandal contributed to the crisis of confidence that affected America throughout the 1970s.