On February 1, 2010, the Defense Department released a report on the 2010 Quadrennial Defense Review, a legislatively mandated assessment of defense strategy, force structure, weapons programs, and operations designed to guide defense programming, operational planning, and budgets projected as far as twenty years ahead. The 2009-2010 exercise is the fourth Quadrennial Defense Review carried out under that name and the sixth comprehensive reexamination of U.S. defense policy since the end of the Cold War. It was preceded by earlier QDRs completed in February 2006, September 2001, and May 1997; by the Bottom-Up Review conducted at the beginning of the Clinton Administration and completed in May 1993; and by the Base Force analysis carried out by the Joint Chiefs of Staff during the George H.W. Bush Administration in 1990 and described in congressional testimony in 1991. These official reassessments have been supplemented by reviews conducted by independently chartered panels, including the 1995 Commission on Roles and Missions of the Armed Forces, the 1997 National Defense Panel that followed the 1997 QDR, and the 1998-1999 Commission on National Security/21 st Century. The 2010 QDR was formally conducted from April 2009, when Secretary of Defense Robert Gates issued guidance on the main tasks of the exercise, through the writing and release of the report on the review on February 1. The QDR report, though quite extensive, is not the main result of the process. Officials who have participated in the review say, rather, that its value lies in having the senior leadership of the Department systematically assess defense strategy, force planning, and programs with a relatively long view ahead. Ultimately, the review resulted in concrete decisions on a broad range of program and policy matters. Much of the 2010 QDR report is devoted to listing initiatives intended to improve capabilities to accomplish critical missions. The QDR also resolved a number of policy matters that have been ongoing topics of discussion within the Defense Department. This CRS report provides an overview of the 2010 QDR, partly with a view toward putting matters the QDR addresses into the context of the ongoing evolution of defense policy, and partly in an effort to identify issues for future QDRs or for a broader review of national security strategy. A recurring question in assessments of the QDR process is to what extent successive QDRs have helped the Defense Department to keep up with changes in the international security environment. Previous QDRs have often been criticized because it has been difficult to trace significant changes in defense policy to the results of each review. If, the critics argue, one of Congress's goals in mandating quadrennial reviews was to encourage a more radical rethinking of defense policy, it is hard to see it in the results of each successive QDR. A contrary view is that, to the extent successive QDRs have redefined global security challenges, changes in force planning in the future may eventually catch up with the evolution of defense strategy. In describing trends in global security, the new QDR differs quite significantly from QDRs prior to 2006, and the 2010 QDR breaks some new ground even compared to its immediate predecessor. As a guide to force programming, however, the outcome of the 2010 QDR appears rather conservative. Through FY2015, the QDR anticipates only minimal changes in force structure, and it does not lay out a force structure beyond the end of the current six-year defense planning period at all. Looking at the cumulative results of post-Cold War defense reviews, beginning with the 1990 Base Force analysis and the 1993 Bottom-Up Review and culminating in the 2010 QDR, the net effect has been a substantial change in the conceptual underpinnings of U.S. defense strategy. In retrospect, the outcome of the 1990 and 1993 reviews was mainly to provide a rationale for not dismantling U.S. military capabilities after the Cold War, as the nation had done following World War I and World War II, and, many argue, after the Vietnam conflict. Both the Base Force analysis and the Bottom-Up Review argued for maintaining essentially a scaled down version of Cold War-era forces. Following the formal dissolution of the Soviet Union, the Bottom-Up Review differed from the Base Force analysis in that it did not identify a requirement to be able to reconstitute forces in the event of a renewed threat. Neither provided a basis for substantially redirecting priorities or for making major changes in the organization of the military services. Reviews at least since the 1995 report of the Commission on Roles and Missions, however, have progressively expanded the range of challenges for which planners see a need to prepare. The four QDRs, in 1997, 2001, 2006, and 2010, can perhaps best be seen as snapshots of an ongoing evolution of strategic thinking away from planning for smaller versions of Cold War-era conventional conflicts, on the model of the 1991 Persian Gulf War, and toward planning to cope with a much more diverse array of challenges and responsibilities. By the time of the 2006 and 2010 QDRs, the basic strategic assumptions guiding military planning had shifted dramatically. One premise is that no future adversary is likely to confront U.S. conventional, Cold War-era military capabilities directly. Instead, any foe, ranging from violent, radical non-state terrorist groups to a technologically advanced near-peer competitor, will try to exploit weaknesses in U.S. defenses through asymmetric means. Moreover, even terrorist groups may use advanced technology to carry out attacks, and competing nations might use irregular means to exploit U.S. vulnerabilities. So the notion of a spectrum of conflict, ranging from unsophisticated insurgents or terrorists at the low end to sophisticated national armies at the high end, is becoming blurred. As Secretary Gates explained, the black-and-white distinction between irregular war and conventional war is an outdated model. We must understand that we face a more complex future than that, a future where all conflict will range along a broad spectrum of operations and lethality. Where near-peers will use irregular or asymmetric tactics and non-state actors may have weapons of mass destruction or sophisticated missiles as well as AK-47s and RPGs. This kind of warfare will require capabilities with the maximum possible flexibility to deal with the widest possible range of conflict. Planners now discuss "hybrid threats" that combine means of attack across the whole spectrum of conflict in almost any confrontation. The proliferation of weapons of mass destruction and the spread of cyberwar capabilities further complicate, and, perhaps, multiply the dangers. A key conclusion of the new QDR is that operations short of a major regional conflict may be at least as demanding, and therefore at least as important in determining the size and composition of the force, as requirements for short-duration, high intensity wars. Operations short of a major regional conflict include what one QDR briefing refers to as "foundational activities," such as permanent and rotational forward deployments and engagement with other nations to prevent and deter conflict. Homeland defense missions, small-scale peacekeeping operations abroad, and foreign military training activities may also cumulatively require the assignment of a substantial number of troops. The QDR report particularly emphasizes the point that long-duration stability or counter-insurgency operations, even if they involve fewer troops than a major conflict, can be especially demanding. Operations such as those in Iraq and Afghanistan require a very large standing force to provide a sufficient rotation base for deploying forces abroad without overly straining combat units and personnel. The 2010 QDR concludes that changes in the global security environment require some adjustments in the balance of investments among elements of the U.S. military force posture. It argues for an emphasis, first of all, on prevailing in current conflicts in Iraq and Afghanistan and against Al Qaeda elsewhere. It also elevates maintaining the quality of the all volunteer force to the level of a strategic objective in itself. It puts particular emphasis on managing the impact on military personnel and their families of a long-duration conflict, with repeated rotations of forces abroad. For future planning, it calls for new initiatives in a number of joint mission areas, including homeland defense, countering weapons of mass destruction, the ability to project power into key regions of the globe in the face of challenges to U.S. access, building the capacity of partner nations to defeat terrorism, and means of ensuring access to cyberspace. At least two key questions follow, however, from the rather radical reassessment of the global security environment in the last two QDRs. The obvious issue is whether new security challenges require not only marginal shifts in the balance among defense programs, but more far-reaching changes in the composition of the force. The 2010 QDR endorses no such major changes in force posture, though it hints that some may become matters of discussion in the future. A second and perhaps even more fundamental issue is to what extent the QDR should address broader trends in the international security environment that affect the role of military power in global affairs and the relationship of military power to other means of U.S. influence. As the QDR report acknowledges, global security relations are being reshaped not just by novel threats, but by many other developments, including the shift of global financial resources to Asia; the growing influence of new powers including the BRIC states, Brazil, Russia, India, and China; the global reach of non-state actors; the further weakening of already fragile states; lowered barriers to acquisition of dangerous technologies, particularly weapons of mass destruction; potential resource scarcity; climate change; demographic trends; and the rapidity with which new diseases can spread. The QDR report discusses how some of these developments may affect defense planning, but it does not generally explain defense planning in the context of broader policies needed to adapt to these trends. Moreover, the QDR report affirms that many global security challenges, including proliferation and terrorism, can only be addressed through multilateral cooperation, but the report discusses collective security largely by reaffirming the value of current regional relationships rather than by discussing the potential value of expanded cooperative security arrangements.