In 1954, a conference in Geneva had divided Vietnam into two parts to bring to an end the French Indo-Chinese War. The terms of the agreement stated that an election would take place, but it never did. The North Vietnamese were determined to reunite Vietnam, and thus helped in the formation of the South Vietnamese terrorist group, the VietCong.
In the aftermath of the Geneva Accords, the United States took on the responsibility of arming and training the troops of South Vietnam. That responsibility expanded, however, and by the time of the assassination of President Kennedy it included 8,000 troops, some of which were accompanying South Vietnamese troops into combat.