He had key roles in planning the Vietnam War, serving as deputy to Paul Nitze at Defense under Kennedy and as Assistant Secretary of State for East Asian and Pacific affairs under Johnson.

After leaving government service in 1969, Bundy served as a historian of foreign affairs, teaching at Massachusetts Institute of Technology (MIT) and at Princeton University, from 1972 to his death.

His book A Tangled Web: The Making of Foreign Policy in the Nixon Presidency (1998) is considered his most important work.

In the early 1950s, Bundy was recruited for the Central Intelligence Agency, serving as an analyst and as chief of staff for the Office of National Estimates.

In 1960, Bundy took a leave of absence from the CIA to serve as staff director for Eisenhower's Commission on National Goals.

After being involved in intelligence and the Council on Foreign Relations, he served from 1961 to 1966 as the National Security Advisor to both the Kennedy and Johnson administrations.

In 1989 he acknowledged the complexity of the presidents' decisionmaking, saying, "In a nutshell, my present feeling is that it was a tragedy waiting to happen, but one made much worse by countless errors along the way, in many of which I had a part.

On October 6, 2000, William Putnam Bundy died at his home in Princeton, New Jersey, at the age of 83 from heart trouble.