Nuclear weapons have a destructive capability unrivaled in the annals of history. Even before the U.S. military first tested them in July 1945, scientists and policy makers recognized that they posed a threat to the continued existence of life on planet Earth, and some struggled valiantly to prevent their use. The first nuclear weapons, although powerful enough to wipe out large parts of the cities of Hiroshima and Nagasaki in Japan and kill hundreds of thousands of people, were relatively small and primitive compared to the weapons that would eventually fill the arsenals of the United States and the Soviet Union. This vast increase in the destructive capability of atomic weapons, however, was not unforeseen. J. Robert Oppenheimer, the scientific director of the Los Alamos, ...