Since the end of the Cold War, the proliferation of weapons of mass destruction has become much more prominent in U.S. national security and foreign policy planning. Revelations about Iraqi, North Korean, South African, and Israeli nuclear weapon programs, the possibility of a nuclear arms race in South Asia, and the multidimensional conflicts in the Middle East all point to the immediacy of this problem. Adding a dangerous new twist is the dissolution of the Soviet Union, a superpower armed with nuclear, chemical, and biological weapons whose successor states are wracked by economic crises and political instability. At least three main factors underlie this renewed emphasis on proliferation. First, the reduced military threat from the former Soviet Union has increased the relative importance of lesser powers, especially if armed with weapons of mass destruction. Second, certain international political and technological trends are increasing the threat to international security from proliferation. Third, new opportunities are opening for enhancing the current international regimes designed to stem proliferation. Since at least as far back as the l96Os, when it sponsored the Non-Proliferation Treaty (NPT), the United States has recognized that proliferation is a global problem and combating it requires high levels of international cooperation. This country has also exerted unilateral influence, successfully in several cases, to discourage proliferation; it will no doubt continue to do so. Nevertheless, placing priority on nonproliferation will require the further development and enforcement of international norms and behavior supporting that objective. International conditions today offer significant opportunities for such cooperation.