On January 31, 2013, during the Senate Armed Services Committee's hearing on the nomination of former Senator Chuck Hagel to be Secretary of Defense, Senator Jeff Sessions questioned Senator Hagel about the Obama Administration's plans for the next steps in nuclear arms control. Specifically, he asked Senator Hagel whether he was committed to honoring the provision in the FY2013 National Defense Authorization Act ( P.L. 112-239 , Section 1282) that requires the Administration to provide briefings to Congress, twice each year, on the status of arms control negotiations with Russia. Senator Hagel responded that he was committed to pursuing the required consultations. Senator Sessions then asked Senator Hagel for a commitment that the Administration would pursue agreements that would lead to further reductions in U.S. nuclear weapons through "the treaty-making power of the President." Specifically, he was seeking assurances that the Obama Administration would not try to bypass the Senate, and its role in providing advice and consent to the ratification of treaties, by reducing U.S. nuclear weapons through unilateral or informal bilateral means. Senator Hagel did not respond to this request. He noted that the President "believes in and is committed to treaties," but he did not accept Senator Sessions' view that future reductions in U.S. nuclear weapons should occur only through the treaty-making process. Senator Sessions' questions, and the concerns voiced by other Members of Congress, respond to both the Obama Administration's stated interest in pursuing further reductions in nuclear weapons and indications in some press reports that the Administration may pursue these reductions, as President George H.W. Bush did in 1991, without a formal treaty. President Obama views New START, which was signed by the United States and Russia in April 2010 and entered into force in February 2011, as "just one step on a longer journey." In his State of the Union Address on February 12, 2013, he pledged that "America will continue to lead the effort to prevent the spread of the world's most dangerous weapons." As a part of this effort, the United States would "engage Russia to seek further reductions in our nuclear arsenals." The United States and Russia have not yet started formal negotiations on further reductions in nuclear weapons. Disagreements about a number of issues, including the U.S. interest in limiting nonstrategic nuclear weapons and Russia's interest in limiting U.S. ballistic missile defense programs, have contributed to this delay. At the same time, congressional concerns about both the Administration's plans to reduce further U.S. nuclear warheads and the magnitude of the Administration's funding requests for the modernization of the U.S. nuclear enterprise have raised questions about whether the Senate would consent to ratification of a new treaty. As a result, many analysts and officials have suggested that the United States and Russia pursue "parallel reductions" based on a mutual understanding, rather than a formal treaty. Press reports indicate that the Administration is considering this approach and might seek an informal understanding, within the framework of the New START Treaty, that reduces current negotiated limits on U.S. and Russian forces. Over the years, the United States has used three mechanisms to reduce its nuclear weapons—formal, bilateral treaties; reciprocal, but informal, understandings; and unilateral adjustments to its force posture. Each of these mechanisms for reducing forces serves different purposes, and each can possess different characteristics for the arms control process. The role of Congress in the arms control process also depends on the mechanism used to reduce forces. The United States signed several formal arms control treaties that limited the numbers of deployed nuclear weapons with the Soviet Union during the Cold War and with Russia in the past two decades. Following Article II of the Constitution, the Senate reviewed these treaties, and, in most cases, voted to provide its advice and consent to ratification. The United States has also reduced its forces unilaterally, with reciprocity from Russia in 1991, when President George H.W. Bush withdrew and eliminated most U.S. shorter range nonstrategic nuclear weapons from bases in Europe and Asia. President Bush did not notify Congress or seek congressional approval before pursuing these reductions. President George W. Bush also withdrew from deployment a number of U.S. nonstrategic nuclear weapons, although he did so without seeking or expecting reciprocity from Russia. He also pursued these reductions without seeking approval from Congress. Several Presidents have reduced unilaterally the number of warheads in the U.S. stored stockpile, as the United States has retired older weapons and responded to changing assessments of the necessary size and structure of the U.S. nuclear force. These changes are a part of the normal force planning process, managed by the Department of Defense and approved by the President, and have also occurred without prior explicit approval from Congress. This report reviews these characteristics and demonstrates their effect on decisions about the use of the different mechanisms. The report begins with a review of the role of nuclear arms control in the U.S.-Soviet relationship, looking at both formal, bilateral treaties and unilateral steps the United States took to alter its nuclear posture. It then turns to the role of arms control in the U.S.-Russian relationship, again reviewing the role of both formal treaties and unilateral measures. The report also describes the role of Congress in the arms control process. It then provides an analytic framework that reviews the characteristics of the different mechanisms, focusing on issues such as balance and equality, predictability, flexibility, transparency and confidence in compliance, and timeliness. Finally the report describes issues that Congress may address as the Obama Administration employs these mechanisms to pursue further reductions in U.S. nuclear weapons. This report does not address the question of whether the United States should pursue further reductions in deployed nuclear weapons, or, if it does, how deep those reductions should be. While many in Congress disagree with the Administration's plans for further reductions, that goal is its stated policy. As a result, the report evaluates different mechanisms that the Administration might use to implement that policy without questioning the underlying policy. In addition, the report will evaluate these mechanisms only in the context of reductions in U.S. and Russian nuclear weapons. It will not evaluate whether other nations—such as China, the United Kingdom, and France—should participate in these reductions or which mechanism would be appropriate if other nations did participate. The United States and Russia deploy far greater numbers of nuclear weapons than these other nations, so they may be able to reduce their weapons further before bringing other nations into the process. During the Cold War, before the demise of the Soviet Union at the end of 1991, arms control played a key role in the relationship between the United States and Soviet Union. Between 1972 and 1991, the two nations signed four treaties and one executive agreement that limited offensive nuclear weapons and ballistic missile defenses. As Table 1 indicates, all but one of these treaties entered into force. Arms control negotiations were often one of the few channels for formal communication between the two nations. The talks provided the United States and Soviet Union with a forum to air their security concerns and raise questions about their plans and programs. Over time, the discussions during negotiations and the data and access mandated by the monitoring provisions included in the treaties allowed for a measure of transparency about the numbers and capabilities of current forces. As the volume of shared information grew over the years, each side could replace suspicions about the intentions of the other with confidence in its understanding of the capabilities of the other's nuclear forces. The limits also helped each side predict and plan for the future size and shape of the other's forces. To most observers, this process reduced the risk of nuclear war and strengthened U.S. security. It helped both sides avoid worst-case assumptions about the future that could fuel an arms race or undermine stability. Others, however, questioned the value of these talks. Some argued that the agreements merely codified existing force structure plans and restricted the U.S. ability to respond to emerging threats. For example, during the 1980s, when the United States renewed its interest and expanded its research into extensive land-based and space-based defenses against ballistic missiles, the 1972 Anti-ballistic Missile (ABM) Treaty continued to limit it to 100 interceptors deployed at one specific location. Some also questioned whether the Soviet Union would comply with its obligations, as the monitoring process revealed evidence of activities that were inconsistent with expectations under the treaties. In spite of predictions to the contrary, however, there was little evidence that the Soviet Union sought to evade the limits in the treaties in any systematic way. Instead, many of the concerns derived from ambiguities in the terms of the treaties and most were resolved in discussions held in compliance review commissions established by the treaties. The United States and Soviet Union also used arms control negotiations, and the resulting treaties, as a way to limit or reduce the specific weapons systems that they viewed as threatening and destabilizing. For example, during the late 1970s and early 1980s, the Soviet Union deployed intermediate range missiles that could reach critical targets in NATO. The United States responded by deploying intermediate-range missiles in Europe; these could have reached leadership and command and control targets in the Soviet Union in less than 10 minutes. Some analysts feared that these weapons would provide the United States and NATO with the ability to "decapitate" Soviet leadership, thereby giving NATO an incentive to use these weapons early in a conflict and the Soviets an incentive to launch its forces quickly before it lost the ability to control its nuclear operations. With both sides fearing a first strike from the other, each had an incentive to reduce the threat. As a result, they negotiated the 1987 Intermediate-range Nuclear Forces (INF) Treaty, which eliminated all intermediate-range nuclear missiles. In a similar vein, throughout the Cold War, U.S. analysts expressed concerns about the Soviet force of large "heavy" intercontinental ballistic missiles (ICBMs) that could often carry multiple warheads (known as multiple independently-targeted reentry-vehicles—MIRVs.)