As the fighting between the Viet Cong and the South Vietnamese continued, the U.S. continued to send additional advisers to South Vietnam. When the North Vietnamese fired directly upon two U.S. ships in international waters on August 2 and 4, 1964 (known as the Gulf of Tonkin Incident), Congress responded with the Gulf of Tonkin Resolution. This resolution gave the President the authority to escalate U.S. involvement in Vietnam. President Lyndon Johnson used that authority to order the first U.S. ground troops to Vietnam in March 1965.

http://history1900s.about.com/od/vietnamwar/a/vietnamwar.htm