Probably the pivotal event in the escalation of American involvement in Vietnam was the overthrow of President Ngo Dinh Diem by a junta of South Vietnamese military officers in the summer of 1963. And that overthrow was instigated not by Kennedy himself, but by a small group of powerful people going behind his back.
As it says in
George Bush: The Unauthorized Biography: "At this time, the group proposing escalation in Vietnam (as well as preparing the assassination of President Diem) had a heavy Brown Brothers, Harriman/Skull and Bones overtone: the hawks of 1961-63 were Harriman, McGeorge Bundy, William Bundy, Henry Cabot Lodge, and some key London oligarchs and theoreticians of counterinsurgency wars. And of course, George Bush during these years was calling for escalation in Vietnam and challenging Kennedy to 'muster the courage' to try a second invasion of Cuba."
http://www.tarpley.net/bush8b.htmThe crucial factor in Diem's overthrow was a cable, sent without Kennedy's knowledge or approval, supporting the anti-Diem plotters:
Although the product of many causes, the US Government's action in 1965 to engage its forces openly and directly in Vietnam can be said to have evolved from mid-1963, when cumulative mistakes by the Ngo Dinh Diem government caused a precipitate decline in South Vietnam's already-shaky performance against its Communist adversary, the Viet Cong.
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The storming of Buddhist pagodas on 21 August by forces directed by Ngo Dinh Nhu crystallized the "Diem must go" convictions, and on Saturday, 24 August, at a time when President Kennedy, National Security Adviser McGeorge Bundy, Secretary of Defense McNamara, Secretary of State Rusk, and DCI McCone happened to be out of town, a small group of strategically placed senior State Department officials smoked a fateful Top Secret/Operational Immediate cable past interagency coordinators to a receptive Ambassador Lodge. In effect, that cable told the Ambassador to advise Diem that immediate steps must be taken to improve the situation--such as meeting Buddhist demands and dismissing his brother. If Diem did not respond promptly and effectively, Lodge was instructed to advise key Vietnamese military leaders that the United States would not continue to support his government.
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Reading the cable only after it had been sent, virtually all of Washington's top officials were critical of the manner in which Hilsman, Harriman, and Forrestal had acted, and in a series of White House meetings the next week the President himself expressed second thoughts about the faults and virtues of the Ngo brothers and the merits of a military coup. . . . Lyndon Johnson later termed the dispatching of the cable a crucial decision that "never received the serious study and detached thought it deserved," a "hasty and ill-advised message" that constituted a green light to those who wanted Diem's downfall, and a "serious blunder which launched a period of deep political confusion in Saigon that lasted almost two years."
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Ambassador Frederick Nolting, displaced in Saigon by Lodge and denigrated in Washington by Hilsman because of his pro-Diem arguments (but whose counsel the President sought in August 1963 to balance that of his detractors), later wrote that in 22 years of public service he had never seen anything "resembling the confusion, vacillation and lack of coordination in the U.S. Government" at that time. Although Nolting had sympathy for President Kennedy, he deplored "his failure to take control" and concluded that "the Harriman-Lodge axis seemed too strong for him."http://ngothelinh.150m.com/CIA_CSI.html