Contrary to many shallow historians, President John F. Kennedy was not going into Vietnam with ground troops; he was pulling 1000 "advisors" out in Dec 1963, all the rest of them by 1965. (see NSAM 263).  He was murdered on Nov 22, 1963, one month before the first 1000 could be pulled out.  Lyndon Johnson reversed this order with NSAM273 four days after the murder and by March 1965 500,000 American soldiers were in Vietnam.

The war in Southeast Asia, as part of the Cold War Strategy, had been in the works during Eisenhower's Presidency, and since JFK had begun to show he was not going to go along with that plan, a cabal of powerful US officials, businessmen and intelligence people in collusion with military leaders, and some foreigners, decided to remove him.