The first months of 1965 saw an escalation of the civil war in South Vietnam, pitting the Communist-led National Liberation Front (or Viet Cong) against the pro-American government of the Republic of Vietnam. The Kennedy administration provided financial aid and military advisers to assist the South Vietnamese, but the lack of either a coherent counter-insurgency strategy or an effective, competent regime in Saigon meant that such assistance was essentially wasted. As the NLF gradually established control over the South, supported with troops and supplies by the Democratic Republic of Vietnam, the Johnson administration decided to launch air-strikes against the DRV to force it to cease assisting its ideological allies in the South. In March 1965 the first US combat units were sent to South Vietnam, and by May American troops were involved in the war against the Viet Cong and supporting North Vietnamese army units. The USA gradually became drawn into the Vietnam War through its determination to arrest the expansion of Communism in South-East Asia, while the North Vietnamese leadership believed that they had been tricked into accepting partition in the 1954 Geneva conference, and were determined to reunify Vietnam by force.

The war in Vietnam had a damaging effect on East-West relations, stalling superpower détente and inflaming Sino-American tensions. The conflict also undermined America's international reputation, as both the bombing campaign against the North (Rolling Thunder) and US military operations in the RVN were condemned by non-aligned governments.