Johnson became President John Kennedy's vice president after serving as Texas governor from 1949 to 1961. He assumed the presidency after Kennedy's assassination in 1963 and became the nation's most powerful proponent for U.S. military interest in Southeast Asia. Following the Tonkin Gulf Resolution, Johnson slowly escalated American involvement in the Vietnam War so that by 1967 more than 500,000 U.S. troops were fighting in the conflict. Johnson eventually began pulling troops out of South Vietnam and decided not to run for re-election in 1968.