1955–1964: Vietnam: First military advisors sent to Vietnam on February 12, 1955. By 1964, U.S. troop levels had grown to 21,000. On August 7, 1964, U.S. Congress approved Gulf of Tonkin resolution affirming "All necessary measures to repel any armed attack against the forces of the United States. . .to prevent further aggression. . . (and) assist any member or protocol state of the Southeast Asian Collective Defense Treaty (SEATO) requesting assistance. . ."[Vietnam timeline]