War Build-Up 1965 was a turning point in the war. With the overthrow of President Diem in 1963, the Southern Saigon government had been weakened and suffered several defeats. Vietcong was expanding, the United States realized that an intervention would be evident. President Johnson ordered the United States to support the Southern Vietnamese people in order to stop the spread of communism. U.S. unit combat buildup began in 1965 and U.S. troops increased to 543,400, holding the record for the longest deployment of U.S. forces.