The Vietnam War: Map of North and South Vietnam, 1955–1975

Vietnam, Laos and Cambodia were ruled by France as French Indochina from the late 19th century; the 1945 button shows it. When Japan surrendered in 1945, the communist-led Viet Minh under Ho Chi Minh declared Vietnam's independence in Hanoi on 2 September. France returned to reclaim its colony, and from 1946 the two fought the First Indochina War. After the communist victory in China in 1949, the Viet Minh received Chinese weapons and advisers, while the United States, fearing the spread of communism, paid a large share of France's war costs.

In 1954 General Vo Nguyen Giap surrounded a French fortress in the remote valley of Dien Bien Phu, near the Laotian border, hauling artillery up the surrounding hills. It fell on 7 May 1954 after a 56-day siege, and France decided to leave.

The Geneva Conference of 1954 ended the French war. Its agreements of 21 July divided Vietnam temporarily along the 17th parallel, at the Ben Hai River, the dashed purple line on the map: the Viet Minh regrouped in the north and French-backed forces in the south, and nationwide elections were to be held in 1956 to reunify the country. In the 300 days during which people could move, up to a million northerners, many of them Catholics, went south.

The elections never took place. In the south, the anti-communist nationalist Ngo Dinh Diem took power, proclaimed the Republic of Vietnam in October 1955 and, with American support, refused to hold the vote, which Ho Chi Minh was expected to win. From 1956 the map shows two Vietnams: the Democratic Republic of Vietnam in the north, with its capital at Hanoi, and the Republic of Vietnam in the south, with its capital at Saigon.

Diem's government was authoritarian and unpopular with many peasants and Buddhists, and from 1959 North Vietnam supported an armed rising in the south. In 1960 the insurgents formed the National Liberation Front, whom their enemies called the Viet Cong. Men and supplies came south along the Ho Chi Minh Trail, a web of jungle tracks through neutral Laos and Cambodia, shown in red. The number of American military advisers rose from about 900 to 16,000 under President Kennedy. In November 1963, with Washington's knowledge, South Vietnamese generals overthrew and killed Diem, and a series of unstable military governments followed.

In August 1964 the US government reported attacks by North Vietnamese torpedo boats on American destroyers in the Gulf of Tonkin; the second attack probably never happened. Congress passed the Gulf of Tonkin Resolution, giving President Lyndon Johnson a free hand. In March 1965 Johnson began the bombing campaign Operation Rolling Thunder against the North and landed the first combat Marines at Da Nang.

By 1968 more than half a million American troops were in Vietnam, alongside a large South Vietnamese army and contingents from South Korea, Australia, New Zealand, Thailand and the Philippines. The US commander, General William Westmoreland, fought a war of attrition, using helicopters, firepower and "search and destroy" missions to kill more enemy soldiers than the North could replace. The Viet Cong and the North Vietnamese army avoided large battles and could withdraw across the borders. American aircraft dropped more bombs on Vietnam, Laos and Cambodia than all sides had used in the Second World War, sprayed the defoliant Agent Orange over forests and crops, and created millions of refugees.

On 30–31 January 1968, during the truce for the Tet lunar new year, about 80,000 Viet Cong and North Vietnamese troops attacked more than 100 towns and cities across South Vietnam, including Saigon, where a squad briefly broke into the US embassy compound. Most attacks were beaten back within days, but the old imperial capital of Hue was held for almost a month and largely destroyed; communist forces executed thousands of its civilians. At the same time the Marine base at Khe Sanh was under siege.

Militarily, Tet was a defeat for the communists, and the Viet Cong lost much of its strength. Politically it was a turning point: after years of official optimism, Americans saw the war on television and lost faith in it. In March 1968 Johnson stopped most of the bombing and announced he would not run for re-election, and peace talks opened in Paris. In the same month American soldiers killed more than 500 unarmed villagers at My Lai, a massacre that became public in 1969.

President Richard Nixon promised "peace with honour". His policy of Vietnamization trained and equipped the South Vietnamese army to take over the fighting while American troops went home, from a peak of about 543,000 in 1969. At the same time he widened the war, invading Cambodia in 1970 to attack the trail and its bases, which set off huge protests at home, and supporting a South Vietnamese incursion into Laos in 1971. North Vietnam's Easter Offensive of 1972 was halted with American air power, and in December 1972 B-52s bombed Hanoi and Haiphong.

The Paris Peace Accords, signed on 27 January 1973, provided for a ceasefire, the withdrawal of the remaining US forces and the return of American prisoners of war, but they left North Vietnamese troops in the South. The last American combat troops left on 29 March 1973.

Without American support, and with US aid cut by Congress, South Vietnam could not hold on. North Vietnam's final offensive began in March 1975 with the capture of Ban Me Thuot in the Central Highlands; the South Vietnamese army's retreat turned into a rout, and Hue and Da Nang fell within weeks. On 29–30 April American helicopters evacuated the last Americans and thousands of Vietnamese from Saigon, and on 30 April 1975 North Vietnamese tanks crashed through the gates of the presidential palace. Saigon was renamed Ho Chi Minh City, and the country was formally reunited as the Socialist Republic of Vietnam on 2 July 1976. In the same year, the Khmer Rouge took Cambodia and the Pathet Lao took Laos.

Estimates of the Vietnamese soldiers and civilians killed range from about 970,000 to 3 million. Some 275,000 to 310,000 Cambodians and 20,000 to 62,000 Laotians died, and 58,220 American service members were killed. Unexploded bombs and the effects of Agent Orange went on killing and maiming for decades. After the war hundreds of thousands of people were sent to re-education camps, and hundreds of thousands fled by sea as "boat people"; the UN refugee agency estimates that 200,000 to 250,000 of them died at sea.

For the United States, defeat brought a crisis of confidence, the end of the military draft and the War Powers Resolution of 1973, which limits a president's power to wage war without Congress. Vietnam and the United States restored diplomatic relations in 1995.