McGeorge Bundy believe that if JFK had not been killed, the Vietnam War "as we know it" would never have happened.

 

McGeorge Bundy, National Security Advisor for both JFK and LBJ, believed that if Kennedy had remained president, the Vietnam War as we know it would never have happened

 QUOTE 

In a discussion with another colleague, James Blight of Brown University, “Mac said there were no missed opportunities,” Blight noted. “None, except the ‘opportunity’ that was irretrievably lost when Kennedy was felled and replaced by Johnson. He said he believes both sides of the argument: had Kennedy lived, there would have been no Vietnam War as we know it; and with Johnson in the White House, it was (in combination with Hanoi’s intransigence) destined to unfold like the tragedy it became.” … 

Bundy argued that the no-combat-troop policy imposed by Kennedy was of fundamental significance for America’s history in Vietnam. “If Lyndon Johnson had not reversed that decision,” Bundy wrote, “the Vietnam War as America came to know could not have happened.” 

UNQUOTE

 [Gordon M. Goldstein, Lessons in Disaster: McGeorge Bundy and the Path to War in Vietnam, p. 231-232]