By 1966 the US had become officially involved in the war in Vietnam. In 1964 the Gulf of Tonkin Resolution, issued by Congress, gave President Johnson the broad power to use military force in Southeast Asia without congressional approval or a formal declaration of war. This resolution therefore greatly increased the war-making power of the president. The resolution was signed after a destroyer, the USS Maddox, was allegedly attacked twice in the Gulf of Tonkin off the coast of North Vietnam. However, the Maddox had been on a secret intelligence mission, and it was revealed that the second attack never actually occurred. Nevertheless, both President Johnson and Congress used this alleged incident to justify increasing US involvement in Vietnam.

The number of US military advisors in Vietnam increased under President Kennedy, and by 1965 there were 200,000 US troops in Vietnam and the draft quota had doubled. In addition to increasing US troop presence, LBJ also increased US bombings of North Vietnam. By 1966 anti-war protests began, mainly led by the Students for a Democratic Society (SDS). These protests would increase and intensify by 1968.

In Johnson's State of the Union Address in 1966, the president attempted to appease these protesters and other opponents of the war. He stated that the US had no territorial or economic ambitions in Vietnam while also reaffirming the United States' commitment to assisting the South Vietnamese fight the North Vietnamese, no matter the cost. Thus LBJ confirmed his belief that the US should stay in Vietnam at a time when war was becoming increasingly unpopular. Throughout 1966, Johnson would continue increasing US troops in Vietnam to 400,000.