"The post-World War II U.S.-Japan alliance has long been an anchor of the U.S. security role in East Asia. The alliance facilitates the forward deployment of about 36,000 U.S. troops and other U.S. military assets in the Asia-Pacific, thereby undergirding U.S. national security strategy in the region. For Japan, the alliance and the U.S. nuclear umbrella provide maneuvering room in dealing with its neighbors, particularly China and North Korea. When a devastating earthquake and tsunami hit Japan on March 11, 2011, U.S.-Japan relations were stable but still recovering from a difficult period in 2009-2010. The Democratic Party of Japan's (DPJ's) landslide victory in the August 2009 elections for the Lower House of Japan's legislature marked the end of an era in Japan; it was the first time Japan's Liberal Democratic Party (LDP) was voted out of office. The LDP had ruled Japan virtually uninterrupted since 1955. Since the resignation of the DPJ's first prime minister, Yukio Hatoyama, in June 2010, bilateral relations have been smoother under the leadership of Naoto Kan and Yoshihiko Noda. The party appears to have shifted its strategic thinking after a series of provocations from North Korea and indications of growing assertiveness from the Chinese military in disputed waters in 2010. The massive and immediate relief provided by the United States following the March 11 disaster bolstered the relationship further."
CRS Report for Congress, RL33436