In early August 1964, two U.S. destroyers stationed in the Gulf of Tonkin in Vietnam radioed that they had been fired upon by North Vietnamese forces. In response to these reported incidents, President Lyndon B. Johnson requested permission from the U.S. Congress to increase the U.S. military presence in Indochina. On August 7, 1964, Congress passed the Gulf of Tonkin Resolution, authorizing President Johnson to take any measures he believed were necessary to retaliate and to promote the maintenance of international peace and security in Southeast Asia. This resolution became the legal basis for the Johnson and Nixon Administrations prosecution of the Vietnam War.
When Johnson asked the U.S. Congress for permission to defend U.S. forces in Southeast Asia, the Senate passed the Gulf of Tonkin Resolution with only two opposing votes, and the House of Representatives passed it unanimously. Congress supported the resolution with the assumption that the president would return and seek their support before engaging in additional escalations of the war.
The Gulf of Tonkin incident and the subsequent Gulf of Tonkin resolution provided the justification for further U.S. escalation of the conflict in Vietnam. Acting on the belief that Hanoi would eventually weaken when faced with stepped up bombing raids, Johnson and his advisers ordered the U.S. military to launch Operation Rolling Thunder and to deploy regular ground combat troops to Vietnam to fight the Viet Cong in the countryside.
Source: U.S. Involvement in the Vietnam War: The Gulf of Tonkin and Escalation, 1964
Office of the Historian, U.S. Department of State