The Vietnam War (1 November 1955 – 30 April 1975) was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam (Democratic Republic of Vietnam) and South Vietnam (Republic of Vietnam) and their allies. North Vietnam was supported by the Soviet Union and China, while South Vietnam was supported by the United States and other anti-communist nations. The conflict was the second of the Indochina wars and a proxy war of the Cold War between the Soviet Union and US. The Vietnam War was one of the postcolonial wars of national liberation, a theater in the Cold War, and a civil war, with civil warfare a defining feature from the outset. Direct US military involvement escalated from 1965 until its withdrawal in 1973. The fighting spilled into the Laotian and Cambodian Civil Wars, which ended with all three countries becoming communist in 1975.

After the defeat of the French Union in the First Indochina War that began in 1946, Vietnam gained independence in the 1954 Geneva Conference but was divided in two at the 17th parallel: the Viet Minh, led by Ho Chi Minh, took control of North Vietnam, while the US assumed financial and military support for South Vietnam, led by Ngo Dinh Diem. The North Vietnamese supplied and directed the Viet Cong (VC), a common front of dissidents in the south which intensified a guerrilla war from 1957. In 1958, North Vietnam invaded Laos, establishing the Ho Chi Minh trail to supply the VC. By 1963, the north had covertly sent 40,000 soldiers of its People's Army of Vietnam (PAVN), armed with Soviet and Chinese weapons, to fight in the insurgency in the south. President John F. Kennedy increased US involvement from 900 military advisors in 1960 to 16,000 in 1963 and sent more aid to the Army of the Republic of Vietnam (ARVN), which failed to produce results. In 1963, Diem was killed in a US-backed military coup, which added to the south's instability.