The U.S.-Japan alliance has long been an anchor of the U.S. security role in Asia and arguably a contributor to peace and prosperity in the region. Forged during 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 54,000 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. Since the early 2000s, the United States and Japan have improved the alliance's operational capability as a combined force, despite constraints. In addition to serving as a hub for forward-deployed U.S. forces, Japan now fields its own advanced military assets, many of which complement U.S. forces in missions like antisubmarine operations. The joint response to a 2011 tsunami and earthquake in Japan demonstrated the two militaries' increased interoperability. Cooperation on ballistic missile defense and new attention to the cyber and space domains remains ongoing. Japanese Prime Minister Shinzo Abe is a stalwart 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. Although constitutional, legal, fiscal, and political barriers hinder further development of defense cooperation, Japan is steadily expanding its capabilities and, subtly, its attitude toward the use of military force, which is constrained by the Japanese constitution. Japan faces a complex security landscape in the region, with potentially significant implications for the alliance. North Korea's increased ballistic missile and nuclear capabilities pose a direct threat to Japan. Both Japan and the United States view China's growing power (especially military power) and territorial assertiveness in the East China Sea and elsewhere as a destabilizing force that diminishes U.S. influence and erodes long-standing norms in the region. Japan has pursued security cooperation with others, including Australia, India, and several Southeast Asian countries, both bilaterally and within the context of the U.S.-Japan alliance. Of particular 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 threats or to influence China's behavior. Limited resources could strain alliance capabilities as well as produce more contentious negotiations on costsharing. The Japanese government currently provides nearly $2 billion per year to offset the cost of stationing U.S. forces in Japan, in addition to purchasing millions of dollars of U.S. defense equipment annually. Furthermore, the alliance has faced new strains in recent years. U.S. President Donald J. Trump's open skepticism of the value of U.S. alliances and his admiration of North Korean leader Kim Jong-un have exacerbated longstanding anxiety in Tokyo about the U.S. commitment to Japan's security.