From the beginning, nuclear weapons have both terrified and fascinated us. Fear of the bomb motivated the first atomic program: the allure of the bomb?s power later propelled national leaders to build ever-larger arsenals. Today, fear of a nuclear attack by terrorists or another country has made nuclear proliferation the number one security threat facing the United States and many other nations. Yet several countries still maintain extensive nuclear arsenals developed for another era, develop plans for new weapons, and postulate new nuclear missions. Leaders in several other nations? motivated by their own perceived security needs as well as by a desire for symbols of power and status?covet the weapons now denied them. This is a book about those weapons. More specifically, it is about how and why nuclear weapons have multiplied, and what can be done to slow, stop, and reverse their spread. This discussion makes one very important assumption: the proliferation of nuclear weapons is undesirable.