United States interest in Vietnam began after the end of World War II, when France attempted to reclaim its colonies in Southeast Asia. France's advances into Vietnam were repelled by the nationalist movement led by Ho Chi Minh, a Communist. After several years of fighting during which France received considerable funding from the United States, France suffered a disastrous defeat at Dienbienphu in 1954 and abandoned Vietnam. The cease-fire agreement split Vietnam into North Vietnam, which was to be ruled by Ho Chi Minh, and South Vietnam, which was to be ruled by Ngo Dihn Diem, who was supported by the United States.

Ho Chi Minh maintained numerous followers in South Vietnam—soon to form a guerrilla force called the Viet Cong—who supported his efforts to reunite the two halves into one country under his rule. As a result of Diem's increasingly repressive regime and resistance to land reform, Ho and the National Liberation Front (NLF), the political arm of the Viet Cong, won increasing support in the South. As Ho's forces acquired new strength, the United States government increased its involvement in South Vietnam, a position that President John F. Kennedy justified as an attempt to prevent the spread of Communism in Southeast Asia. By the time of Kennedy's assassination in November 1963, about 16,000 troops had been sent to Vietnam. American troops (officially called advisers) began to assume a more active role that included sabotage, espionage, and orchestrating Southern Vietnamese attacks on the North.

Initially, the Tonkin Gulf resolution strengthened public support for American intervention in Vietnam. Johnson and his advisers took advantage of this support to adopt new, aggressive strategies. In early 1965, after a Viet Cong attack on American troops at Pleiku, Johnson ordered the first in a series of sustained bombings of North Vietnam. The bombings, called Operation Rolling Thunder, were initially intended to end the traffic of soldiers and weapons along the Ho Chi Minh Trail, but soon the missions were expanded. By the end of the war the United States had dropped nearly 7 million tons of bombs on Vietnam. Although the United States government said that it targeted only Viet Cong military bases, many planes actually bombed civilian villages, Buddhist temples, and churches as well, and the vast majority of people killed by the strikes or burned by napalm were village women, children, and the elderly.

Following the launch of Operation Rolling Thunder, Gen. William Westmoreland, the United States Army commander in Vietnam, requested that Johnson send American combat troops into the conflict. The first troops landed in Da Nang in March 1965; by 1968 the number of American troops in Vietnam had surpassed 500,000. The troops were used to escalate the war on the ground and were frequently sent on “search-and-destroy” missions to villages suspected of collaborating with the Viet Cong. Frequently the missions entailed surrounding a village, killing the men of military age, burning the homes and food, and ordering the surviving villagers to refugee camps.

As the war progressed, the United States adopted increasingly drastic measures. In the late 1960s the Central Intelligence Agency (CIA) initiated the Phoenix Program, which was aimed at destroying the Viet Cong infrastructure by “neutralizing” its supporters; the program led to the executions, without trial, of about 20,000 South Vietnamese who were suspected of collaborating with the Viet Cong. In addition, United States military advisers supervised and participated in routine torture and beatings of Vietnamese prisoners of war. The troops themselves also began to adopt harsher methods. In March 1968, for example, American soldiers massacred the unarmed inhabitants of a village named My Lai.

Despite these efforts, however, the United States was unable to defeat the NLF. In fact, throughout the war, the NLF was able to maintain high morale and popular support throughout Vietnam. This support emboldened the Viet Cong to organize a surprise attack on Saigon and other South Vietnamese cities and towns in January 1968. The so-called Tet offensive enabled the Viet Cong to briefly occupy Saigon, including the American embassy. American troops forced the Viet Cong out of Saigon within several days, and Johnson ordered retaliatory bombings in North Vietnam. Nevertheless, the attack had made clear the fragility of the United States position in Vietnam and prompted the start of peace talks with the NLF.

The Tet offensive also intensified the antiwar movement in the United States, which had developed soon after the beginning of the conflict. As early as 1964, citizens, including prominent artists, musicians, and academics, had started to criticize the war, and young men eligible for the draft had begun to burn their draft cards in protest. Soon many draftees, including heavyweight boxer Muhammad Ali, refused to report for service when called. As the war progressed and details were published by journalists and international observers, war protests became commonplace throughout the United States, especially on college campuses. The demonstrations frequently provoked violent reactions from the authorities; at Kent State University in Ohio in 1970, the National Guard fired into a crowd of protestors, killing four students.

Opposition to the war was further strengthened by the publication of the ‘Pentagon Papers' in 1971. The documents, which contained excerpts of defense department reports on the war, exposed lies told by the United States government to the American people concerning its intentions in Vietnam.

By the early 1970s, American popular opinion firmly opposed the war, and morale in Vietnam was failing. Nevertheless, Nixon initially made only token gestures toward ending the war. Although he promised “peace with honor,” ended the draft, and reduced the number of American soldiers in Vietnam, he simultaneously expanded the war into Cambodia and Laos. A massive attack by the Viet Cong on South Vietnam in April 1972, however, finally prompted Nixon to take the peace talks seriously. In the summer of 1972 he sent Secretary of State Henry Kissinger to Vietnam to negotiate a cease-fire with North Vietnam. Finally, in late March 1973, the last American combat troops were airlifted out of Saigon. The war ended two years later when South Vietnam fell to Ho Chi Minh and his Communist forces.