The end of the Cold War left the United States in the fortunate position of facing no imminent threat of global war. But it also left the United States in a strategic vacuum, with no organizing principle for its national security. This book proposes a security strategy for the 21st century based on preventing new major threats to U.S. security from emerging. Informed by the authors' service in the Pentagon during President Clinton's first term, this book identifies six major dangers to U.S. security that have the potential to grow into threats to American interests and values as ominous as the Cold War's nuclear standoff. In chapters that cover chilling dangers ranging from Russia's implosion to the rising power of China, and from proliferation of biological weapons to cyber terrorism, the authors first recount from first hand experience the Pentagon's efforts to define and prevent dangers to U.S. security since the end of the Cold War, and then advance preventive defense strategies for the future. It argues that implementing a Preventive Defense strategy will require a revolution in the way the Pentagon does business -- a revolution that is only beginning.