The president had the authority to increase American forces in South Vietnam
following the Gulf of Tonkin Resolution.
When the United States Congress passed the Gulf of Tonkin Resolution on August 7, 1964 in response to the Gulf of Tonkin incident, it granted the president free rein to escalate military intervention eventually to over 500,000 troops. It gave U.S. President Lyndon B. Johnson authorization, without a formal declaration of war by Congress, for the utilization of conventional military force in Southeast Asia.