While deterrence is as old as human conflict itself, it became particularly important with the advent of nuclear weapons when armed conflict between the superpowers had the potential to end civilization. Today there is a sense that terrorism has rendered deterrence obsolete and forced the United States to substitute preemption for it. The author illustrates that strategic reality is not simple. Instead, the two are inextricable. He provides both a conceptual framework for understanding deterrence or, more accurately, the psychology of deterrence and policy guidance on how the United States can most effectively use it. The author concludes that an adaptable and flexible military with robust landpower is the only tool that can maintain deterrence.