Nuclear weapons, despite their unprecedented destructiveness, proved to be politically stabilizing forces during the years of Cold War from 1947 until the fall of the Berlin Wall in 1989. In the twenty-first century nuclear weapons will be primarily destabilizing forces. This book explains the reasoning behind both assertions. The argument about the destabilizing potential of nuclear weapons is not entirely new. The dangers offered by the actual and potential spread of nuclear weapons are apparent to some informed observers. Similar dangers existed in the Cold War and will continue to do so as long as states and non-state actors have built-in grievances against the existing order, or against one another.