in august 2008, the world observed the sixty-third anniversary of the atomic bombings of Hiroshima and Nagasaki, history's first and only instances of nuclear attack. Since 1945, the nuclear age has generated considerable debate on the utility, morality, and legitimacy of the acquisition, possession, control, and use of nuclear weapons. Buried in the global discourse on the nuclear dilemma is the fact that atomic weapons have not been used militarily by any nuclear state since 1945. While their non-use between nuclear states may largely be explained by the operation of mutual deterrence, it remains a puzzle as to why these weapons have not been used against nonnuclear opponents, who could not retaliate in kind. In some cases, nuclear weapon states (NWS) have lost the wars they fought against nonnuclear weapon states (NNWS). Occasionally, NWS have experienced the temptation to use their atomic arms, yet desisted from doing so. In other instances, nonnuclear states initiated crises or wars, anticipating non-use of atomic weapons by the defending nuclear state. In some other cases, NNWS continued fighting with NWS, and thereby imposed enormous costs on the latter in personnel and resources, even though the NWS had the capability to retaliate with nuclear weapons and thus terminate the war expeditiously.
I argue that the unwillingness to use nuclear weapons can be partially attributed to an informal norm inherent in the tradition of non-use, which has gradually emerged since 1945. A tradition in this sense is a time-honored practice of non-use that has been followed by nuclear states since 1945 as an “accustomed obligation.”1 This tradition has largely been shaped by two