The first half century of the nuclear age witnessed the gradual construction of a global nuclear order designed to mitigate nuclear dangers, inhibit arms racing, and prevent the spread of nuclear weapons to additional states. Spurred by the experiences, the dangers, the crises, the near misses, and the frightening risks on display in the early years of the Cold War, sustained efforts were made, in McGeorge Bundy’s vivid phrase, “to cap the volcano.” The time had arrived, Bundy wrote in 1969, for the two great nuclear superpowers “to limit their extravagant contest in strategic weapons,” a contest that had “led the two greatest powers of our generation into an arms race totally unprecedented in size and danger.” In the subsequent twenty-five years after Bundy’s appeal, an increasingly elaborate and institutionalized arms control process produced, with many ups and downs, a detailed web of constraints on the nuclear behavior of the superpowers. The articulated goal was to stabilize the superpower nuclear balance by reinforcing mutual deterrence. The vast nuclear arsenals of the superpowers, however, were not the only source of nuclear danger. In a world in which the number of states armed with nuclear weapons was slowly growing and many additional states had interest in acquiring such weapons or the technology to produce them, there was reason, as Albert Wohlstetter warned in 1961, to be “concerned with the enormous instabilities and dangers of a world with many nuclear powers.” Such a world—“life in a nuclear armed crowd”—Wohlstetter wrote in a later famous study, was widely believed to be “vastly more dangerous than today’s world.” The desire to prevent this unattractive world led to the negotiation of the Nuclear Nonproliferation Treaty (NPT), which entered into force in 1970, and to the subsequent development of an associated regime intended to create legal and technical barriers to the spread of nuclear weapons. Thus, in reaction to the major perceived dangers of the nuclear age, there emerged what Lawrence Freedman calls the “twin pillars” of the global nuclear order: mutual stability in the major nuclear rivalry and nonproliferation to inhibit or prevent the spread of nuclear weapons to additional states.