With the end of the Cold War and the disintegration of the USSR from 1989 to 1991, the bipolar balance of terror suddenly collapsed, and it became clear that the Soviet nuclear strength had been disguising severe internal weakness. Almost overnight, it seemed, the international environment had changed almost beyond recognition. But the U.S. and NATO defense postures, built up so carefully and at such expense over the previous half century, could not change so quickly; U.S. foreign and military policies relied heavily on deterrence and they would need time to adjust. For deterrence, this had profound and often unforeseen challenges.
As it happened, cutting forces was relatively straightforward; the more difficult stage of adapting military strategy to the post–Cold War situation was that of reducing reliance on nuclear weapons. The problem was approached in two steps. First, the progress made over the previous decade in arms reduction was to be consolidated and advanced. The United States and Russia were committed to deep cuts in their strategic arsenals; under the terms of the START (Strategic Arms Reduction Talks) II treaty, those arsenals would be reduced to approximately one-third their size at the height of the Cold War, and both sides would eliminate the most destabilizing of the first-strike weapons, the MIRVed ICBMs. NATO, still formally committed to the defense posture of flexible response, concluded that it had to move away from a forward defense posture and that, accordingly, in the post–Warsaw Pact strategic landscape, substrategic, short-range nuclear weapons—that is, tactical IRBMs and MRBMs—no longer had a viable deterrent role. Consequently, at the NATO heads of state meeting in London on 5–6 July 1990, NATO committed itself to eliminating all nuclear artillery shells in Europe. At the same time NATO declared that it now regarded nuclear forces as "truly weapons of last resort." Simultaneously, negotiations were under way for what became the Conventional Forces in Europe (CFE) Treaty, which provided for drastic cuts in both NATO and Warsaw Pact conventional forces stationed in Central Europe. In September 1991, President George H. W. Bush declared that the forward deployment of tactical nuclear forces was no longer a useful part of the U.S. deterrent and that he was therefore ordering the removal of all tactical nuclear weapons from the U.S. Navy. Never fully abandoning Reagan's dream of an impenetrable shield against incoming missiles, the Bush administration quietly proceeded with a scaled-down version of SDI.
If the end of the Cold War drastically reduced the likelihood of strategic nuclear war, it nevertheless increased the risk of a small-scale nuclear exchange, mainly because of the growing problem of nuclear proliferation. In an effort to bring nuclear policy up to date and to confront head-on the problem that nuclear proliferation and the equalization of power that it created might one day work against the United States, the Clinton administration in late 1993 announced that it planned to redefine "deterrence." No longer would the emphasis be on preventing the use of weapons of mass destruction, since that risk had declined markedly; instead, the focus would be on preventing the acquisition of those weapons. The announcement was followed up in September of the following year with a formal replacement of the MAD doctrine with "mutual assured safety" (MAS), a long-term program designed primarily to make Russia's military reductions irreversible by reducing not only the number of weapons themselves but also reducing the technological and industrial infrastructures needed for nuclear weapons development. Through economic incentives and technological aid, steps were taken to dismantle what Soviet leader Mikhail Gorbachev called the "infrastructure of fear." By November 1997, in the first formal presidential directive on the actual employment of nuclear weapons since the Carter administration, President Clinton formally abandoned the Cold War tenet that the U.S. military forces must be prepared to fight a protracted nuclear war. Nuclear weapons would still play an important deterrent role, but the emphasis on them would be reduced in keeping with the changing nature of the threats in the post–Cold War international environment.
While the role of nuclear weapons in the U.S. military posture was reducing, the role of conventional weapons was increasing. Confronting crises in the Balkans and the Middle East, the United States and its allies demonstrated that they could now project conventional military force with great effectiveness. Exploiting the so-called "revolution in military affairs" of superior intelligence information coupled with technologically advanced conventional weapons, the United States and NATO were able to strike with overwhelming conventional military force in a precise and controlled manner, leading to successful combat in both regions while suffering few casualties. Such capability, demonstrated convincingly and publicly, became an important part of U.S. efforts to confront and deter what the defense community called "asymmetrical threats," or threats from rogue nations or terrorist organizations.
The role played by nuclear weapons and the deterrence strategies they bred since 1945 is a controversial issue. Many historians have argued that the very existence of nuclear weapons deterred the outbreak of another global war and kept what the historian John Lewis Gaddis called "the long peace" since 1945, while others have argued that other factors rendered major wars obsolete and that nuclear weapons were a largely irrelevant factor. Many others have argued that the sometimes absurd military postures that the existence of nuclear weapons encouraged greatly and needlessly increased the risk of global destruction and that the peace was maintained despite the existence of nuclear weapons. But whether a positive, negative, or irrelevant force, deterrence has made indelible impressions on the practice of foreign policy and the public imagination.