More than sixty years have passed since the American use of atomic bombs on Hiroshima and Nagasaki, the only use of nuclear weapons in warfare. The non-use of nuclear weapons since then remains the single most important phenomenon of the nuclear age. Yet we lack a full understanding of how this situation arose and is maintained, and of its prospects for the future. As military historians have noted, it is rare for a weapon found useful on one occasion to remain unused in the next. Such an outcome was not inevitable. At the height of the nuclear arms race in the 1980s, nearly 70,000 nuclear weapons existed in the world's nuclear arsenals. In addition, an extensive array of military plans and organizations, national policies, public commitments, and alliances all contemplated the employment of such weapons. Many reasonable observers expected that nuclear weapons would be used at some point during the Cold War. It was thus by no means inevitable that, after their use against Japan in August 1945, a “tradition” of non-use would arise.