Ellsberg began working as a strategic analyst at the RAND Corporation for the summer of 1958 and then permanently in 1959.[6] He concentrated on nuclear strategy, working with leading strategists such as Herman Kahn and challenging the existing plans of the United States National Security Council and Strategic Air Command.[7]
Ellsberg completed a PhD in economics from Harvard in 1962.[5] His dissertation on decision theory was based on a set of thought experiments that showed that decisions under conditions of uncertainty or ambiguity generally may not be consistent with well-defined subjective probabilities. Now known as the Ellsberg paradox,[8] it formed the basis of a large literature that has developed since the 1980s, including approaches such as Choquet expected utility and info-gap decision theory.[9]
Ellsberg worked in the Pentagon from August 1964[10] under Secretary of Defense Robert McNamara as special assistant to Assistant Secretary of Defense for International Security Affairs John McNaughton. He then went to South Vietnam for two years, working for General Edward Lansdale as a member of the State Department.[11]
On his return from South Vietnam, Ellsberg resumed working at RAND. In 1967, he contributed with 33 other analysts to a top-secret 47-volume study of classified documents on the conduct of the Vietnam War, commissioned by Defense Secretary McNamara and supervised by Leslie H. Gelb and Morton Halperin.[12][13][14] These 7,000 pages of documents, completed in late 1968 and presented to McNamara and Clark Clifford early in the following year, later became known collectively as the "Pentagon Papers".[15][14][13]
