Answer:
to prevent the Vietcong from overthrowing the South Vietnamese government
Step-by-step explanation:
At that time, Vietnam involved in the middle of the cold war between 2 ideologies. Capitalism & Democracy which led by United States and Communism & Dictatorships which led by the Soviet Union. The Soviet Union managed to got a strong foothold on Northern Vietnam while United States backed Southern Vietnam.
Kennedy believed that if they let the Soviet Union managed to helped North Vietnam to overthrew South Vietnam, communism will rapidly spread to other countries in Asia Pacific. This is why he initiate U.S' involvement in Vietnam.