The Vietnam War (also known by other names) 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. The north was supported by the Soviet Union, China, and other communist states, while the south 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 officially becoming communist states by 1976.

With the defeat of the French Union in the First Indochina War and its acceptance of military withdrawal from Vietnam pursuant to the Geneva peace agreement on Vietnam that took effect on 23 July 1954, the country gained the independence from France but was divided into two military gathering areas: the Viet Minh took control of North Vietnam, while the U.S. assumed financial and military support for the South Vietnamese state. The Viet Cong (VC), a South Vietnamese common front under the direction of the north, initiated a guerrilla war in the south. The People's Army of Vietnam (PAVN), also known as the North Vietnamese Army (NVA), engaged in more conventional warfare with U.S. and Army of the Republic of Vietnam (ARVN). North Vietnam invaded Laos in 1958, establishing the Ho Chi Minh Trail to supply and reinforce the VC. By 1963, the north had sent 40,000 soldiers to fight in the south.  U.S. involvement increased under President John F. Kennedy, from just under a thousand military advisors in 1959 to 23,000 by 1964.

Following the Gulf of Tonkin incident in August 1964, the U.S. Congress passed a resolution that gave President Lyndon B. Johnson broad authority to increase U.S. military presence in Vietnam, without a formal declaration of war. Johnson ordered the deployment of combat units for the first time, and dramatically increased the number of American troops to 184,000.[62] U.S. and South Vietnamese 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, and continued significantly building up its forces, despite little progress being made. In 1968, North Vietnamese forces launched the Tet Offensive; though it was a military defeat for them, it became a political victory, as it caused U.S. domestic support for the war to fade.  By the end of the year, the VC held little territory and were sidelined by the PAVN. In 1969, North Vietnam declared the Provisional Revolutionary Government of the Republic of South Vietnam. Operations crossed national borders, and the U.S. bombed North Vietnamese supply routes in Laos and Cambodia. The 1970 deposing of the Cambodian monarch, Norodom Sihanouk, resulted in a PAVN invasion of the country (at the request of the Khmer Rouge), and then a U.S.-ARVN counter-invasion, escalating the Cambodian Civil War. After the election of Richard Nixon in 1969, a policy of "Vietnamization" began, which saw the conflict fought by an expanded ARVN, while U.S. forces withdrew in the face of increasing domestic opposition. U.S. ground forces had largely withdrawn by early 1972, and their operations were limited to air support, artillery support, advisors, and materiel shipments. The Paris Peace Accords of January 1973 saw all U.S. forces withdrawn  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 to the PAVN on 30 April, marking the end of the war; North and South Vietnam were reunified on 2 July the following year.

The war exacted an enormous human cost: estimates of the number of Vietnamese soldiers and civilians killed range from 966,000[21] 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. The end of the Vietnam War would precipitate the Vietnamese boat people and the larger Indochina refugee crisis, which saw millions of refugees leave Indochina, an estimated 250,000 of whom perished at sea. Once in power, the Khmer Rouge carried out the Cambodian genocide, while conflict between them and the unified Vietnam would eventually escalate into the Cambodian–Vietnamese War, which toppled the Khmer Rouge government in 1979. In response, China invaded Vietnam, with subsequent border conflicts lasting until 1991. Within the United States, 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.

The U.S. Air Force destroyed more than 20% of the jungles of South Vietnam and 20–50% of the mangrove forests by spraying over 20 million gallons of toxic herbicides (defoliants) including Agent Orange. The war is one of the most commonly used examples of ecocide.