A coup by some of his own generals succeeded in toppling and killing Diem and his brother, Ngo Dinh Nhu, in November 1963, three weeks before Kennedy was assassinated in Dallas, Texas. The ensuing political instability in South Vietnam persuaded Kennedy’s successor, Lyndon B. Johnson, and Secretary of Defense Robert McNamara to further increase U.S. military and economic support. The following August, after DRV torpedo boats attacked two U.S. destroyers in the Gulf of Tonkin, Johnson ordered the retaliatory bombing of military targets in North Vietnam. Congress soon passed the Gulf of Tonkin Resolution, which gave Johnson broad war-making powers, and U.S. planes began regular bombing raids, codenamed Operation Rolling Thunder, the following February.

In March 1965, Johnson made the decision–with solid support from the American public–to send U.S. combat forces into battle in Vietnam. By June, 82,000 combat troops were stationed in Vietnam, and General William Westmoreland was calling for 175,000 more by the end of 1965 to shore up the struggling South Vietnamese army. Despite the concerns of some of his advisers about this escalation, and about the entire war effort as well as a growing anti-war movement in the U.S., Johnson authorized the immediate dispatch of 100,000 troops at the end of July 1965 and another 100,000 in 1966. In addition to the United States, South Korea, Thailand, Australia and New Zealand also committed troops to fight in South Vietnam (albeit on a much smaller scale).