Vietnam War, conflict in Southeast Asia, primarily fought in South Vietnam between government forces aided by the United States and guerrilla forces aided by North Vietnam. The war began soon after the Geneva Conference provisionally divided (1954) Vietnam at 17? N lat. into the Democratic Republic of Vietnam (North Vietnam) and the Republic of Vietnam (South Vietnam). It escalated from a Vietnamese civil war into a limited international conflict in which the United States was deeply involved, and did not end, despite peace agreements in 1973, until North Vietnam's successful offensive in 1975 resulted in South Vietnam's collapse and the unification of Vietnam by the North.

In part, the war was a legacy of France's colonial rule, which ended in 1954 with the French army's catastrophic defeat at Dienbienphu and the acceptance of the Geneva Conference agreements. Elections scheduled for 1956 in South Vietnam for the reunification of Vietnam were canceled by President Ngo Dinh Diem. His action was denounced by Ho Chi Minh, since the Communists had expected to benefit from them. After 1956, Diem's government faced increasingly serious opposition from the Viet Cong, insurgents aided by North Vietnam. The Viet Cong became masters of the guerrilla tactics of North Vietnam's Vo Nguyen Giap. Diem's army received U.S. advice and aid, but was unable to suppress the guerrillas, who established a political organization, the National Liberation Front (NLF) in 1960.

In 1961, South Vietnam signed a military and economic aid treaty with the United States leading to the arrival (1961) of U.S. support troops and the formation (1962) of the U.S. Military Assistance Command. Mounting dissatisfaction with the ineffectiveness and corruption of Diem's government culminated (Nov., 1963) in a military coup engineered by Duong Van Minh; Diem was executed. No one was able to establish control in South Vietnam until June, 1965, when Nguyen Cao Ky became premier, but U.S. military aid to South Vietnam increased, especially after the U.S. Senate passed the Tonkin Gulf resolution (Aug. 7, 1964) at the request of President Lyndon B. Johnson.

In early 1965, the United States began air raids on North Vietnam and on Communist-controlled areas in the South; by 1966 there were 190,000 U.S. troops in South Vietnam. North Vietnam, meanwhile, was receiving armaments and technical assistance from the Soviet Union and other Communist countries. Despite massive U.S. military aid, heavy bombing, the growing U.S. troop commitment (which reached nearly 550,000 in 1969), and some political stability in South Vietnam after the election (1967) of Nguyen Van Thieu as president, the United States and South Vietnam were unable to defeat the Viet Cong and North Vietnamese forces. Optimistic U.S. military reports were discredited in Feb., 1968, by the costly and devastating Tet offensive of the North Vietnamese army and the Viet Cong, involving attacks on more than 100 towns and cities and a month-long battle for Hue in South Vietnam.

Serious negotiations to end the war began after U.S. President Lyndon Johnson's decision not to seek reelection in 1968. Contacts between North Vietnam and the United States in Paris in 1968 were expanded in 1969 to include South Vietnam and the NLF. The United States, under the leadership of President Richard M. Nixon, altered its tactics to combine U.S. troop withdrawals with intensified bombing and the invasion of Communist sanctuaries in Cambodia (1970).

The length of the war, the high number of U.S. casualties, and the exposure of U.S. involvement in war crimes such as the massacre at My Lai helped to turn many in the United States against the war. Politically, the movement was led by Senators James William Fulbright, Robert F. Kennedy, Eugene J. McCarthy, and George S. McGovern; there were also huge public demonstrations in Washington, D.C., as well as in many other cities in the United States and on college campuses.

Even as the war continued, peace talks in Paris progressed, with Henry Kissinger as U.S. negotiator. A break in negotiations followed by U.S. saturation bombing of North Vietnam did not derail the talks, and a peace agreement was reached, signed on Jan. 27, 1973, by the United States, North Vietnam, South Vietnam, and the NLF's provisional revolutionary government. The accord provided for the end of hostilities, the withdrawal of U.S. and allied troops (several Southeast Asia Treaty Organization countries had sent token forces), the return of prisoners of war, and the formation of a four-nation international control commission to ensure peace.

Fighting between South Vietnamese and Communists continued despite the peace agreement until North Vietnam launched an offensive in early 1975. South Vietnam's requests for aid were denied by the U.S. Congress, and after Thieu abandoned the northern half of the country to the advancing Communists, a panic ensued. South Vietnamese resistance collapsed, and North Vietnamese troops marched into Saigon Apr. 30, 1975. Vietnam was formally reunified in July, 1976, and Saigon was renamed Ho Chi Minh City. U.S. casualties in Vietnam during the era of direct U.S. involvement (1961?72) were more than 50,000 dead; South Vietnamese dead were estimated at more than 400,000, and Viet Cong and North Vietnamese at over 900,000.