Analysing documents produced by the CIA, the State Department and the Pentagon, the thesis examines the role of intelligence assessment in U.S. Vietnam policy during the period between December 1961 and February 1965. It investigates intelligence on the counterinsurgency in South Vietnam, on the intentions and capabilities of North Vietnam, and on the probable consequences of policy options. The first half of the thesis examines the Vietnam intelligence during the Kennedy administration, following the rise of optimism in 1962 and the intelligence dispute in 1963. The second half of the study explores intelligence developments from the fall of the Diem regime in November 1963 to President Johnson’s decision to take military action against North Vietnam in February 1965. The study suggests that intelligence deficiencies played a significant role in both the failure of counterinsurgency in the first half of the 1960s and in the decision for direct military intervention in 1965. The thesis also demonstrates that, rather than simply being a result of technical weaknesses, the lack of robust intelligence reflected wider problems of Vietnam policy, including political pressures, ideological contexts and the absence of strategic consensus.