This essay takes a history-of-ideas approach to place American medicine and psychiatry within the context of the Cold War. Rather than focus on physical science and technology (as is often the case in studies of science and the Cold War), the essay shifts attention to the human sciences in order to consider the ways in which medical and psychiatric research was caught up in concerns over national defence. Various arguments have proposed that US research involving human subjects between the late 1940s and the mid-1960s contravened the Nuremberg Code, which was established in 1948 to formalize the ethical boundaries of medical research and prevent any repeat of the human experimentation widely practised in Nazi concentration camps. The essay focusses, particularly, on two phases of the Cold War: the medical and genetic testing linked to the dropping of the atom bomb in Hiroshima and Nagasaki and the psychological tests (often referred to as “psyops”) beginning in the World War II years but developing rapidly during the Korean War. The aim of the essay is to link a phenomenological approach to the Cold War – in which the figures of the “zero” and “cipher” frequently arose – with a discussion of the interrelationship between US national defence issues and American medical research.