	Introduction

The U.S.-Japan alliance, forged in the U.S. occupation of Japan after its defeat in World War II, provides a platform for U.S. military readiness in Asia. Under the Treaty of Mutual Cooperation and Security, about 50,000 U.S. troops are stationed in Japan. The United States has the exclusive use of 85 facilities throughout the archipelago, providing the major U.S. forward logistics base in the Asia-Pacific region; Okinawa hosts 33 of the facilities. A series of provocations by North Korea and increasingly aggressive maritime operations by China since 2010 appeared to have set the relationship back on course. Emboldened by its own economic growth and a perception of U.S. decline, Beijing has asserted itself more forcefully in diplomatic and military arenas, including direct challenges to Japan's territorial rights over a set of islets in the East China Sea. The U.S.-Japan alliance, missing a strategic anchor since the end of the Cold War, may have found a new guiding rationale in shaping the environment for China's rise. A Rejuvenated Alliance Since 2012

Prime Minister Abe's commitment to defense reform has dovetailed effectively with the Obama Administration's drive to upgrade bilateral alliances as party of the "strategic rebalancing" to the Asia-Pacific region. Specifically, Abe has adjusted Japan's interpretation of its constitution to allow for the exercise of the right of collective self-defense, passed a package of security legislation that provides a legal framework for the new interpretation, slightly increased Japan's defense budget, relaxed Japan's restrictions on arms exports, established a National Security Council to facilitate decisionmaking on foreign policy, passed a "State Secrets" bill that allows for more intelligence-sharing with the United States, and committed political capital and resources to advance the U.S.-Japan agreement to relocate a controversial U.S. Marine Corps airbase in Okinawa. Many of these initiatives still face considerable obstacles, but the momentum has created new energy in the alliance. The United States and Japan have been steadily enhancing bilateral cooperation in many aspects of the alliance, such as ballistic missile defense, cybersecurity, and military use of space. As Japan sheds its self-imposed restrictions on the use of military force (in particular the constraints on collective self-defense) and the two countries implement their revised bilateral defense guidelines, the opportunities for the U.S. and Japanese militaries to operate as a combined force will grow. North Korea's belligerent rhetoric and repeated ballistic missile tests have heightened the sense of threat in Japan. The U.S.-Japan alliance has been a vehicle for enhancing security ties with Southeast Asian countries, especially since maritime territorial disputes in the East and South China Seas began to intensify in the late 2000s. Without cooperation among its allies, the United States may find itself less able to respond to North Korean nuclear and missile threats and to influence China's behavior. The United States has pursued new basing arrangements with countries in Southeast Asia that could host rotations of troops or other assets: Singapore, the Philippines, Australia, and Malaysia have, to varying degrees, allowed or indicated a willingness to provide expanded access to the United States, although the vast majority of U.S. military assets in Asia will remain in Japan and South Korea for the foreseeable future. In recent years, Japan has donated dozens of used and new patrol boats to the coast guards in the region. Increased joint training activities and shared use of facilities has improved the interoperability of the U.S.-Japan alliance. Japan also plays an active role in extended deterrence through its BMD capabilities. Forces Japan

The Japanese government provides nearly $2 billion per year to offset the cost of stationing U.S. forces in Japan (see Figure 3 ). About 40% of all facilities used by U.S. Forces Japan (USFJ) and half of USFJ military personnel are located in the prefecture, which comprises less than 1% of Japan's total land area. Because of these widespread concerns among Okinawans, the sustainability of the U.S. military presence in Okinawa remains a challenge for the alliance.
The U.S.-Japan alliance has long been an anchor of the U.S. security role in Asia. Forged in the U.S. occupation of Japan after its defeat in World War II, the alliance provides a platform for U.S. military readiness in the Pacific. About 50,000 U.S. troops are stationed in Japan and have the exclusive use of 85 facilities. In exchange for the use of these bases, the United States guarantees Japan's security. Security challenges in the region, particularly nuclear and missile tests by North Korea and increased Chinese maritime activities, have reinforced U.S.-Japan cooperation in recent years. The vitality of the alliance is particularly salient as the Obama Administration renewed the U.S. focus on the Asia-Pacific region through a strategic "rebalancing." The U.S.-Japan alliance, missing a strategic anchor since the end of the Cold War, may have found a new guiding rationale in shaping the environment for China's rise.
Since the early 2000s, the United States and Japan have taken significant strides in improving the operational capability of the alliance as a combined force, despite constraints. In addition to serving as hub for forward-deployed U.S. forces, Japan fields its own advanced military assets, many of which complement U.S. forces in missions like anti-submarine operations. The joint response to a 2011 tsunami and earthquake in Japan demonstrated the interoperability of the two militaries. Cooperation on ballistic missile defense and new attention to the cyber and space domains has also been strong. Japan's own defense policy has evolved, and major strategic documents reflect a new attention to operational readiness and flexibility.
Steady progress on an initiative to realign U.S. forces based in Japan has been overshadowed by the failure to resolve difficult basing issues on Okinawa, the major U.S. forward logistics base in East Asia. About 40% of all facilities used by U.S. Forces Japan (USFJ) and half of USFJ military personnel are located in the prefecture, which comprises less than 1% of Japan's total land area. The sustainability of the U.S. military presence on Okinawa remains a critical challenge for the alliance. The long-delayed plan to relocate Marine Corps Air Station Futenma from a densely populated area of Okinawa encountered further obstacles in the first half of 2016.
Japanese Prime Minister Shinzo Abe is a strong supporter of the alliance and has had notable success on his ambitious agenda to increase the capability and flexibility of Japan's military. Abe's dominance over Japanese politics since his election in late 2012 has created opportunities for more predictable alliance planning. However, constitutional, legal, fiscal, and political barriers prevent a significant expansion of defense cooperation. Many of Abe's initiatives have faced opposition from the public and from political parties. In addition, leaders in China and South Korea distrust Abe because of his past statements on Japanese actions in the World War II era. Suspicion from Beijing and Seoul complicates Japan's efforts to expand its security role.
Japan faces a complex security landscape in the region. North Korea's increased asymmetric capabilities pose a direct threat to Japan. A territorial dispute with China over a set of islets in the East China Sea raises the risk of military escalation, a scenario that could trigger U.S. treaty obligations to defend Japan. Japan has pursued security cooperation with others in the region, including Australia, India, and several Southeast Asian countries. Of concern to the United States is the tense Japan-South Korea relationship, which has prevented effective trilateral coordination. Without cooperation among its allies, the United States may find itself less able to respond to North Korean missile threats and to influence China's behavior.
Both Japan and the United States face significant fiscal challenges. Limited resources could strain alliance capabilities as well as produce more contentious negotiations on cost-sharing. The Japanese government provides nearly $2 billion per year to offset the cost of stationing U.S. forces in Japan.