Lyndon B. Johnson (LBJ), as he took over the presidency after the death of Kennedy, initially did not consider Vietnam a priority and was more concerned with his "Great Society" and progressive social programs. Presidential aide Jack Valenti recalls, "Vietnam at the time was no bigger than a man's fist on the horizon. We hardly discussed it because it was not worth discussing."

On November 24, 1963, Johnson said, "the battle against communism... must be joined... with strength and determination." The pledge came at a time when Vietnam was deteriorating, especially in places like the Mekong Delta, because of the recent coup against Diem.

Johnson had reversed Kennedy's disengagement policy from Vietnam in withdrawing 1,000 troops by the end of 1963 (NSAM 263 on Oct. 11),^[118] with his own NSAM 273 (Nov. 26) to expand the war.

The military revolutionary council, meeting in lieu of a strong South Vietnamese leader, was made up of 12 members headed by General Duong Van Minh—whom Stanley Karnow, a journalist on the ground, later recalled as "a model of lethargy." Lodge, frustrated by the end of the year, cabled home about Minh: "Will he be strong enough to get on top of things?" His regime was overthrown in January 1964 by General Nguyen Khanh. However, there was persistent instability in the military as several coups—not all successful—occurred in a short space of time.

On August 2, 1964, the USS Maddox, on an intelligence mission along North Vietnam's coast, allegedly fired upon and damaged several torpedo boats that had been stalking it in the Gulf of Tonkin.

A second attack was reported two days later on the USS Turner Joy and Maddox in the same area. The circumstances of the attack were murky. Lyndon Johnson commented to Undersecretary of State George Ball that "those sailors out there may have been shooting at flying fish."

The second attack led to retaliatory air strikes, prompted Congress to approve the Gulf of Tonkin Resolution, and gave the president power to conduct military operations in Southeast Asia without declaring war. In the same month, Johnson pledged that he was not "... committing American boys to fighting a war that I think ought to be fought by the boys of Asia to help protect their own land."

An undated NSA publication declassified in 2005, however, revealed that there was no attack on August 4. It had already been called into question long before this. "The Gulf of Tonkin incident", writes Louise Gerdes, "is an oft-cited example of the way in which Johnson misled the American people to gain support for his foreign policy in Vietnam." George C. Herring argues, however, that McNamara and the Pentagon "did not knowingly lie about the alleged attacks, but they were obviously in a mood to retaliate and they seem to have selected from the evidence available to them those parts that confirmed what they wanted to believe."

"From a strength of approximately 5,000 at the start of 1959 the Viet Cong's ranks grew to about 100,000 at the end of 1964...Between 1961 and 1964 the Army's strength rose from about 850,000 to nearly a million men."^[108] The numbers for U.S. troops deployed to Vietnam during the same period were quite different; 2,000 in 1961, rising rapidly to 16,500 in 1964.

The National Security Council recommended a three-stage escalation of the bombing of North Vietnam. On March 2, 1965, following an attack on a U.S. Marine barracks at Pleiku, Operation Flaming Dart, Operation Rolling Thunder and Operation Arc Light commenced. The bombing campaign, which ultimately lasted three years, was intended to force North Vietnam to cease its support for the National Front for the Liberation of South Vietnam (NLF) by threatening to destroy North Vietnam's air defenses and industrial infrastructure. As well, it was aimed at bolstering the morale of the South Vietnamese. Between March 1965 and November 1968, "Rolling Thunder" deluged the north with a million tons of missiles, rockets and bombs.

Bombing was not restricted to North Vietnam. Other aerial campaigns, such as Operation Commando Hunt, targeted different parts of the NLF and Vietnam People's Army (VPA) infrastructure. These included the Ho Chi Minh Trail, which ran through Laos and Cambodia. The objective of forcing North Vietnam to stop its support for the NLF, however, was never reached. As one officer noted "this is a political war and it calls for discriminate killing. The best weapon... would be a knife... The worst is an airplane."^[133] The Chief of Staff of the United States Air Force Curtis LeMay, however, had long advocated saturation bombing in Vietnam and wrote of the Communists that "we're going to bomb them back into the Stone Age".