In early August 1964, North Vietnamese forces fired upon two U.S. destroyers stationed in the Gulf of Tonkin in Vietnam. In response, 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. The resolution authorized 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’ actions during the Vietnam War.
Congress passed the resolution on the assumption that the president would seek their support before escalating the war further.
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. Johnson and his advisers believed that Hanoi would eventually weaken when faced with increased bombing raids. The president ordered the U.S. military to launch Operation Rolling Thunder, which deployed 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