The United States fought a major war in Vietnam during the 1960s and early 1970s.
The Vietnam War was a Cold War conflict involving North Vietnam and South Vietnam, with the Soviet Union and China supporting the North and the United States backing the South. Although fighting began earlier, direct American military involvement escalated sharply after 1964, following the Gulf of Tonkin incident. U.S. troop numbers reached about 536,000 by 1969.
American forces relied on air power, large-scale firepower, and search-and-destroy operations, while North Vietnamese and Viet Cong forces used guerrilla tactics and extensive networks of jungle and rural bases. The 1968 Tet Offensive became a major psychological and political turning point in the United States, despite being a military setback for communist forces.
The war officially involved Vietnam, but fighting also spread into neighboring Laos and Cambodia. U.S. combat troops were withdrawn after the 1973 Paris Peace Accords, and North Vietnam captured Saigon in 1975. Laos and Cambodia were involved in related conflicts, but Vietnam is the canonical answer to this question.