The nuclear taboo has been a powerful force inhibiting US resort to use of nuclear weapons since World War II. No taboo existed in 1945, but from the Korean War, when an emerging taboo entered deliberations mostly as an instrumental consideration, to the 1991 Gulf War, when it had become more embedded and internalized, it has restrained use of nuclear weapons, both by appearing as a “constraint” to actors and by engendering more constitutive processes of stigmatization and categorization. Despite cases where nuclear weapons were perceived to have military utility, US leaders have ruled out their use for political and normative reasons. Ultimately, in delegitimizing nuclear weapons, the nuclear taboo has constrained the practice of “self-help” in the international system.