"Under the Treaty of Mutual Cooperation and Security, about 53,000 U.S. troops are stationed in Japan and have the exclusive use of 89 facilities throughout the archipelago. In exchange for the bases, the United States guarantees Japan's security. The alliance has endured over 50 years, through periods of intense partnership and stretches of political drift. In the past decade, the relationship has seen both ends of the spectrum. During the first term of the George W. Bush Administration, converging U.S. and Japanese objectives in confronting North Korea's nuclear and missile programs and Japan's participation in U.S. operations in Iraq and Afghanistan reinforced the notion of the U.S.-Japan alliance as one of the central partnerships of U.S. foreign policy, particularly in Asia. By 2007, political developments in Japan and diverging policy approaches to North Korea created some distance in the relationship. After the Democratic Party of Japan took power in a historical election in September 2009, a disagreement over the relocation of the Futenma Marine airbase in Okinawa erupted into a public rift that led many to question the fundamental soundness of the alliance. Regional developments in 2010, however, appeared to refocus attention in Washington and Tokyo on the value of the alliance. North Korea's continued and increasingly aggressive actions, coupled with a diplomatic crisis after a Chinese trawler rammed a Japanese Coast Guard ship in disputed waters, drove the allies back together. A new DPJ administration in Tokyo affirmed its intent to work out U.S. base realignment issues and renewed its financial support for hosting the troops. At the same time, solidarity grew in confronting North Korea provocations."
CRS Report for Congress, RL33740