The Bush Administration described the strategy of "tailored deterrence" to define the role that nuclear weapons might play in U.S. national security policy. Yet, there has been little discussion of this concept, either in Congress or in the public at large, that would allow this concept to serve as a mechanism to help determine the size and structure of the U.S. nuclear force. This absence of discussion has also made it difficult to identify the ways in which tailored deterrence differs, if at all, from the Cold War concept of strategic deterrence and how it might alter the role of nuclear weapons in U.S. national security strategy. Moreover, as it completes its own nuclear posture review early in 2010, the Obama Administration may further refine and revise the rationale for U.S. nuclear weapons and the concepts it uses to describe U.S. deterrence strategy and nuclear policy. This report highlights the differences between the construct of tailored deterrence and the more general concept of strategic deterrence that guided U.S. nuclear policy during the Cold War. It then identifies a number of issues that Congress might address when it reviews these differences, including the question of whether detailed and tailored attack plans are more likely to enhance deterrence or more likely to lead to the early use of nuclear weapons, and the question of whether tailored deterrence provides any guidance about the future size and structure of U.S. nuclear forces. Nuclear weapons were at the center of U.S. national security policy for more than 50 years. From the end of World War II, and, particularly, from the first explosion of a Soviet nuclear weapon in 1949, until the end of the Cold War in 1991, the United States relied on nuclear weapons to deter Soviet aggression and forestall the outbreak of a global war between the United States and the Soviet Union. Generally, the United States sought to maintain "nuclear and conventional capabilities sufficient to convince any potential aggressor that the costs of aggression would exceed any potential gains that he might achieve." But the Soviet Union was (and Russia remains) the only nation that could pose a global challenge to U.S. allies and interests and threaten to cause massive destruction in the United States. Other nations, such as China and Soviet allies in Eastern Europe, were included in the U.S. nuclear war plans, but the Soviet threat dominated U.S. defense planning. Nuclear forces were sized to deter the Soviet threat; these were then thought to be sufficient to deter or respond to the "lesser included cases" of threats from other nations. Although there were often debates about the numbers and types of weapons that the United States should deploy in its nuclear arsenal, there was little doubt, or debate, among analysts, experts, and government officials about the need for the United States to deter the Soviet threat. This widespread consensus about the nature of the threat to the United States and its allies, and the need for nuclear weapons to deter and respond to this threat, began to dissolve during the 1990s, after the demise of the Soviet Union. Some began to argue that, in the absence of the threat of global nuclear war, the United States should declare that nuclear weapons would serve only as "weapons of last resort" to deter nuclear attacks, or possibly other catastrophic attacks, against the United States itself. Others argued that the United States needed to maintain a credible nuclear deterrent to "hedge" against the possibility of a resurgence of the Russian threat. Still others argued that the United States should craft a nuclear policy and force posture that would allow it deter and/or respond to the threat of nuclear, chemical, or biological attacks from a growing number of potential adversaries. Although many analysts outside government hoped the United States would sharply limit the role of nuclear weapons after the end of the Cold War, the Clinton Administration did not adopt a more restrictive, or "last resort," posture for U.S. nuclear weapons. Throughout the 1990s, the Clinton Administration argued that, in addition to serving as a hedge against the potential resurgence of a Russian threat, nuclear weapons remained important to deter the range of threats faced by the United States. It highlighted, in its 1999 National Security Strategy, that "the United States must continue to maintain a robust triad of strategic forces sufficient to deter any hostile foreign leadership with access to nuclear forces and to convince it that seeking a nuclear advantage would be futile." Furthermore, nuclear deterrence would not necessarily be limited to potential aggressors with nuclear weapons. Although the United States did not explicitly threaten the use of nuclear weapons against non-nuclear states, Assistant Secretary of Defense Edward Warner did indicate in testimony before Congress that "the very existence of U.S. strategic and theater nuclear forces, backed by highly capable conventional forces, should certainly give pause to any rogue leader contemplating the use of WMD [weapons of mass destruction; i.e., nuclear, chemical and biological weapons] against the United States, its overseas deployed forces, or its allies." The Bush Administration shifted U.S. nuclear doctrine, at least rhetorically, away from concerns about a resurgent Russian threat. It declared that the United States and Russia no longer view each other as adversaries, and it stated, when releasing the results of its nuclear posture review (NPR) in early 2002, that the United States "will no longer plan, size or sustain its nuclear forces as though Russia presented merely a smaller version of the threat posed by the former Soviet Union." It has, instead, emphasized that nuclear weapons "provide credible capabilities to deter a wide range of threats, including weapons of mass destruction and large-scale conventional military force." Some analysts consider this shift to be more rhetorical than real, as the United States maintains nuclear forces with much of the same size and shape as it did during the 1990s. Nevertheless, the Bush Administration has continued to emphasize the need to size and shape U.S. nuclear forces to deter emerging threats from a growing number of potential adversaries. Specifically, the Bush Administration has indicated that the United States would employ a strategy of "tailored deterrence," where the weapons and attack strategies guiding U.S. nuclear forces would be "tailored" to address the specific capabilities and goals of emerging adversaries. Although the Obama Administration has not yet completed its nuclear posture review or enunciated a role for nuclear weapons in U.S. national security strategy, reports indicate it will continue to focus on the emerging threats from nuclear proliferation, weapons of mass destruction, and the possible use of a nuclear weapon by non-state actors. However, where the Bush Administration planned to counter these threats by leaving open the option to retaliate with nuclear weapons, the Obama Administration, according to some reports, may propose that the United States enhance its intelligence, interdiction, and nonproliferation efforts. Most discussions about U.S. nuclear weapons policy, whether they occur in Congress or in the public and academic literature, focus on how many and what types of weapons the United States should deploy to implement its nuclear doctrine and deterrent strategy. The discussions have also addressed questions about whether the United States should pursue the design and development of new types of nuclear warheads, either to extend U.S. nuclear capabilities or to provide an alternative means of maintaining the U.S. arsenal and U.S. nuclear capabilities into the future. But a growing number of analysts and officials agree that it will be difficult to answer questions about the future size and structure of the U.S. nuclear arsenal without at least some agreement about the role this arsenal should play in U.S. national security policy. The former director of the National Nuclear Security Administration (NNSA), Ambassador Linton Brooks, noted this development during remarks he made to a conference in January 2007, when he stated that "we are increasingly hearing from thoughtful observers that political support for the Reliable Replacement Warhead (RRW) ... will not be possible without greater consensus on the future role of nuclear weapons." Several committees in Congress have also requested reports from the Administration that would link U.S. nuclear weapons strategy to plans for U.S. nuclear weapons and the nuclear weapons infrastructure. This perspective has led to calls for a national debate on the future role of nuclear weapons in U.S. national security strategy. It has also served as the backdrop for numerous studies completed in 2008, in anticipation of the presidential transition, that reviewed the U.S. nuclear posture and offered recommendations for changes in the size and structure of the U.S. nuclear force. Although the specific recommendations vary, most of these studies recommend that the United States reduce the number of deployed forces and limit the role of nuclear weapons in U.S. national security strategy. These questions will be addressed, again, in the Obama Administration's nuclear posture review. Many national security and defense documents produced in the last few years emphasize how much the national security environment for the United States has changed since the demise of the Soviet Union in 1991 and the terrorist attacks in 2001. The report of the 2006 Quadrennial Review, for example, noted that the United States now faces an international security environment "characterized by uncertainty and surprise," and that it must transform its national security strategy in response to this new environment. Specifically, and of interest for this report, the Defense Department identified the need to shift "from 'one size fits all' deterrence to tailored deterrence for rogue powers, terrorist networks, and near-peer competitors." The Pentagon's emphasis on the distinction between "one size fits all" deterrence and tailored deterrence was designed to evoke an understanding of just how much the nature of the threat and the number of U.S. adversaries have changed since the end of the Cold War. But, even during the Cold War, the United States tailored its nuclear targeting doctrine, its nuclear weapons employment policy, and its nuclear force structure to enhance or maintain the credibility of its nuclear deterrent posture.