This report presents background information and discusses potential issues for Congress relating to U.S. ballistic missile defense (BMD) efforts in the Asia-Pacific region. These efforts pose several potential policy, funding, and oversight issues for Congress. Decisions that Congress makes on these issues could affect U.S. defense funding requirements and military capabilities, and U.S. relations with countries in the Asia Pacific region, including China, Japan, South Korea, North Korea, and Australia. This report focuses on U.S. BMD efforts specific to the Asia-Pacific area. Other CRS reports cover U.S. BMD efforts elsewhere and issues other than BMD affecting U.S. relations with countries in the Asia-Pacific region. The growing number and modernization of ballistic missiles in the Asia-Pacific region poses a security challenge for the United States and its allies. Observers believe North Korea has a large arsenal of short-range ballistic missiles (SRBMs). These North Korean SRBMs are believed to have sufficient range to hit targets in South Korea and Japan, including some U.S. military bases there. In recent years, North Korea has also conducted several tests of a long-range ballistic missile system that culminated in a successful space launch in December 2012. Yet, North Korea has not to date demonstrated a reliable capability to hit targets such as Guam or other U.S. territory with a ballistic missile. Congress has expressed strong concern about the ballistic missile threat from both North Korea and Iran and strong interest in ballistic missile defense (BMD) systems to counter those threats. Section 229 of the National Defense Authorization Act (NDAA) for FY2013 ( P.L. 112-239 ) states that it is the sense of the Congress that "the threat from regional ballistic missiles, particularly from Iran and North Korea, is serious and growing, and puts at risk forward-deployed forces, assets, and facilities." The Department of Defense (DOD) is seeking to counter this threat with various measures, including the deployment of increasingly capable BMD systems. Although China is not the focus of U.S. BMD policy, Chinese missiles nevertheless present a complicating factor and increasing challenge for U.S. policy. As a matter of policy and as a result of treaty commitments, the United States extends deterrence to protect its allies in the Asia-Pacific region. In essence, this means the United States will help deter threats to these allies and, if deterrence fails, use U.S. assets to defeat these threats. In 2010, the United States publicly confirmed to South Korea—and thus implicitly to Australia, Japan, and other allies covered by the U.S. "nuclear umbrella"—that extended deterrence includes BMD capabilities. Congress reinforced this commitment in the FY2013 NDAA when it stated in Section 229 that, "The United States has an obligation to meet its security commitments to its allies, including ballistic missile defense commitments." Similarly, the FY2014 NDAA ( P.L. 113-66 ) and FY2015 NDAA ( P.L. 113-291 ) encourage the United States to cooperate with regional allies on BMD issues to enhance the security of all partners. As the threat from ballistic missiles has increased, the United States has gradually expanded its deployment of BMD assets and associated sensors in East Asia. The configuration of sensors, command-and-control (C2) centers, and BMD interceptors in East Asia—in other words, the regional "architecture" of U.S. BMD—has slowly evolved in concert with contributions from treaty allies. Cooperation on regional BMD offers the potential for greater effectiveness and cost efficiency, but it is proceeding at different rates with different countries. The U.S.-Australia partnership on early warning satellites dates back to the early Cold War and the Defense Support Program (DSP) that began in 1970. The United States and Japan have been cooperating on BMD programs since the 1990s and have a mature partnership. South Korea and Australia are beginning to acquire the necessary hardware and software for a more robust BMD capability to include missile interceptors. Southeast Asian allies Thailand and the Philippines have so far not spent their relatively scarce defense funds to procure and deploy BMD systems. The most authoritative DOD directive on BMD policy, the 2010 Ballistic Missile Defense Review Report (BMDR), declared a policy of implementing the "phased adaptive approach" (PAA) in East Asia. That approach seeks to address the most acute near-term threats with deployments of existing technology and to pursue the deployment of BMD programs designed to deal with future, longer range ballistic missile threats as those threats develop. New capabilities are phased in to the system as they become available. The implementation of the phased adaptive approach in East Asia would require more formal arrangements. Fulfilling the requirement in Section 229 of the FY2013 NDAA, in August 2013 DOD published a report to Congress on regional ballistic missile defense that elaborates on the "phased, tailored, and adaptive approaches" to regional BMD architectures. Although the BMDR and the 2010 Nuclear Posture Review (NPR) both explicitly commit the United States to the goal of maintaining "strategic stability" with Russia and China, the two countries have raised strong objections to U.S. BMD programs. Moscow and Beijing both argue that the programs pose a threat to their nuclear deterrents, with Russian concerns focused on BMD in Europe, and Chinese concerns focused on BMD in the Asia-Pacific. Issues for Congress related to the evolution of U.S. BMD posture and policy in the Asia-Pacific region include appropriations for BMD programs; the potential for Foreign Military Sales (FMS) financing of BMD technology to allies; the role of BMD cooperation in shaping alliance relationships and overall U.S. strategy in the Asia-Pacific region; the effect of U.S. BMD cooperation on U.S. relations with China, North Korea, and Russia; and the possible role of U.S. BMD cooperation in influencing Chinese, North Korean, and Russian military developments. The stated focus of U.S. BMD policy is to defend against limited missile strikes from so-called rogue states, namely Iran and North Korea, on the U.S. homeland or against allies and U.S. forces deployed abroad. As a matter of policy, U.S. missile defenses are not intended to alter the balance of nuclear deterrence with the major nuclear-armed states, i.e. Russia and China. U.S. and allied BMD assets in the Asia-Pacific region are, however, inherently capable of intercepting short-range ballistic missiles (SRBMs) in East Asia that are launched not just from North Korea, but also from China. Future U.S. BMD capabilities in the region may be designed to counter much longer-range ballistic missiles regardless of their country of origin. The guiding policy for deployment of BMD capabilities under the Obama Administration has been the Phased Adaptive Approach (PAA). The PAA seeks to address the most acute near-term threats with deployments of existing technology and to pursue BMD programs designed to deal with future, longer-range ballistic missile threats as those threats develop. U.S. policy aims to develop and deploy an overall global BMD system eventually linking regional and homeland BMD. The 2010 BMDR states that the United States "will pursue a phased adaptive approach within each region [Europe, the Persian Gulf, and East Asia] that is tailored to the threats unique to that region, including their scale, the scope and pace of their development, and the capabilities available and most suited for deployment." Regional BMD systems can provide a number of strategic benefits to the United States both in peacetime and in war, and can help to prevent premature escalation of a crisis into a war. In 2009, the United States announced plans for a European PAA and has so far completed the first phase of that effort. Phase two is on track with the installation of an Aegis Ashore system in Romania in 2015. Efforts to formalize an Asia-Pacific PAA (APPAA) are underway, but prospects remain unclear. Implementation of an APPAA could provide an opportunity for the United States to identify and prioritize missile threats and to rally allies and partners around a common vision for defending their territory and U.S. deployed forces in the region. Currently, many of the platforms and sensors required for a Phase 1 APPAA are already in place, such as Patriot and Aegis interceptor platforms and various ground-based sensors and radars. What is lacking is a formal agreement that would lay out a path forward for a more integrated approach with allies and partners, as well as rules of engagement and clarity on various command and control issues. In late 2011, the Obama Administration announced a new centerpiece for its foreign and defense policy known as the "strategic rebalancing" (originally called a "pivot") to the Asia-Pacific region . The January 2012 Defense Strategic Guidance issued by DOD also highlights this change, and U.S. officials have since expounded on the various elements of the strategy. Since 2012, the U.S. military has increased rotational deployments to Australia, the Philippines, and South Korea, and has pledged to deploy its most advanced assets (such as the F-35 Joint Strike Fighter) to East Asia at an early date. One major thrust of the rebalancing strategy is deepened engagement with allies and partners in the region and a concurrent effort to knit strong bilateral ties into a web of regional security cooperation, particularly among treaty allies. The United States has fostered U.S.-Australia-Japan and U.S.-Japan-South Korea trilateral security cooperation and has encouraged India to take a more proactive role in Asia-Pacific security. Some Chinese observers see these developments as a policy to "contain" China, although U.S. officials and many non-Chinese scholars reject that notion.