What happened
US involvement grew from advisors in the 1950s to combat troops from 1965, after the Gulf of Tonkin incident and resolution of 1964; by 1969 over 500,000 US troops were deployed, alongside massive bombing campaigns north and south. Heavy use of chemical defoliants, including Agent Orange, caused long-term harm still documented today.
The 1968 Tet Offensive shattered US confidence; documented atrocities include the My Lai massacre of the same year. The Paris Peace Accords of January 1973 withdrew US forces, and Saigon fell on April 30, 1975.
Background
The 1954 Geneva Accords had divided Vietnam at the 17th parallel pending elections that never came, and the conflict between the two Vietnams drew in the United States and its allies.
Consequences
Vietnamese deaths are commonly estimated between one and three million, most of them civilians; about 58,000 US troops died. Reunification followed in 1976, the postwar years saw the exodus of the boat people, and the US and Vietnam normalized relations only in 1995.