William P. Bundy (1917-2000) was Assistant Secretary of Defense for International Security Affairs during the Kennedy Administration and Assistant Secretary of State for Far Eastern Affairs during the Johnson Administration. He was the elder brother of McGeorge Bundy, who was Assistant to the President for National Security Affairs in both Administrations. Both Bundys were involved in policymaking with respect to the Vietnam War, William at a lower level than McGeorge.
He had served in U.S. intelligence positions starting in the Second World War. After earning a master's degree in history from Harvard, he entered Harvard Law School in 1950, but enlisted in the U.S. Army in 1941, and was assigned to the British ULTRA program of communications intelligence against Nazi Germany. For his work at Bletchley Park, he was awarded the U.S. Legion of Merit and was made a member of the Order of the British Empire. While he finished a law degree and briefly practiced law, he was recruited by the Central Intelligence Agency (CIA) during the Korean War. At CIA, he led a staff preparing intelligence estimates, and then prepared papers for the Eisenhower Administration. 
At the State Department, he was characterized as "the pivot" of the State Department's Vietnam policy. He was the one "who had contact with the secretary and under secretary, while at the same time the lower-level men, the experts, had 90 percent of their contact with him."
Robert McNamara wrote that Bundy helped McNamara and Maxwell Taylor draft recommendations to Lyndon Johnson, in September 1963, offered only 50-50 odds that a replacement of Diem would improve the situation. . They did not recommend a coup be supported.Later, Bundy assessed the Gulf of Tonkin incident as
Miscalculation by both the U.S. and North Vietnam is, at the end, at the root of the best hindsight hypothesis of Hanoi's behavior. In simple terms, it was a mistake for an Administration sincerely resolved to keep its risks low to have had the 34A operations and the destroyer patrol take place even in the same time period. Rational minds could not readily have foreseen that Hanoi might confuse them...but rational calculations hould have taken account of the irrational.
- Martin, Douglas (October 7, 2000), "William P. Bundy, 83, Dies; Advised 3 Presidents on American Policy in Vietnam", New York Times
- Halberstam, David, The Best and the Brightest
- Robert S. McNamara (1995), In Retrospect: the Tragedy and Lessons of Vietnam, Times Books division of Random House, pp. 77-79
- William Bundy, Vietnam Manuscript, quoted in Robert S. McNamara, In Retrospect, pp. 140-141