As the daily headlines attest, the Department of Defense is in the midst of a Quadrennial Defense Review (QDR). Charged by Congress, the Department of Defense is examining a broad range of issues concerning U.S. military policy and strategy (inter alia, future national defense strategy, the force structure necessary to implement that strategy, the affects of technology on force structure, and the anticipated roles and missions of the Reserve Components in executing the defense strategy) that will have far-reaching consequences for the United States. Before these crucial issues are addressed, however, a more fundamental question needs to be explored: what does the United States want its military to do? In other words, what are the future roles of the U.S. military? Only after this issue has been answered can the Department of Defense turn to the other important issues posed by Congress. Dr. William T. Johnsen tackles this question. In brief, he concludes that the U.S. military will continue to perform its traditional roles: deterrence, reassurance, compellence, and support to the nation. The method and manner of carrying out those roles, however, will change; in some cases substantially. The implications of these adapted roles will be considerable. More importantly, Dr. Johnsen also examines the emerging role of preventive defense and its potentially profound consequences for the U.S. military. The debates carried out within and about the QDR will shape the security policy of the United States well into the 21st Century.