South Vietnam, officially the Republic of Vietnam (RVN; Vietnamese: Vit Nam Cng Ha; French: Rpublique du Vit Nam), was a country in Southeast Asia that existed from 1955 to 1975, the period when the southern portion of Vietnam was a member of the Western Bloc during part of the Cold War. It first received international recognition in 1949 as the State of Vietnam within the French Union, with its capital at Saigon (renamed to Ho Chi Minh City in 1976), before becoming a republic in 1955. South Vietnam was bordered by North Vietnam to the north, Laos to the northwest, Cambodia to the southwest, and Thailand across the Gulf of Thailand to the southwest. Its sovereignty was recognized by the United States and 87 other nations, though it failed to gain admission into the United Nations as a result of a Soviet veto in 1957.The end of the Second World War saw anti-Japanese Vit Minh guerrilla forces, led by communist fighter Ho Chi Minh, proclaiming the establishment of the Democratic Republic of Vietnam in Hanoi in September 1945. In 1949, anti-communist politicians formed a rival government in Saigon led by former emperor Bo i. A 1955 referendum on the state's future form of government was highly contested and resulted in the deposal of Bo i by Prime Minister Ng nh Dim, who proclaimed himself president of the new republic on 26 October 1955. After the 1954 Geneva Conference, it abandoned its claims to the northern part of the country and established its sovereignty over the southern half of Vietnam consisting of Cochinchina (Nam K) a former French colony and parts of Annam (Trung K) a former French protectorate. Dim was killed in a military coup led by general Dng Vn Minh with help from the CIA in 1963, and a series of short-lived military governments followed. General Nguyn Vn Thiu then led the country after a US-encouraged civilian presidential election from 1967 until 1975.

The beginnings of the Vietnam War occurred in 1955 with an uprising by the newly organized National Liberation Front for South Vietnam (Vit Cng), armed and supported by the North Vietnam, with backing mainly from China and the Soviet Union. Larger escalation of the insurgency occurred in 1965 with American intervention and the introduction of regular forces of Marines, followed by Army units to supplement the cadre of military advisors guiding the Southern armed forces. A regular bombing campaign over North Vietnam was conducted by offshore US Navy airplanes, warships, and aircraft carriers joined by Air Force squadrons through 1966 and 1967. Fighting peaked up to that point during the Tet Offensive of February 1968, when there were over a million South Vietnamese soldiers and 500,000 US soldiers in South Vietnam. What started as a guerrilla war eventually turned into a more conventional fight as the balance of power became equalized. An even larger, armored invasion from the North commenced during the Easter Offensive following US ground-forces withdrawal, and had nearly overrun some major northern cities until beaten back.

Despite a truce agreement under the Paris Peace Accords, concluded in January 1973 after five years of on-and-off negotiations, fighting continued almost immediately afterwards. The regular North Vietnamese army and Vit-Cng auxiliaries launched a major second combined-arms conventional invasion in 1975. Communist forces overran Saigon on 30 April 1975, marking the end of the Republic of Vietnam. On 2 July 1976, North Vietnam-controlled Provisional Revolutionary Government of the Republic of South Vietnam and the Democratic Republic of Vietnam (North Vietnam) merged 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.