"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. U.S.-Japan relations have been adjusting to the Democratic Party of Japan's (DPJ) landslide victory in the August 30, 2009, elections for the Lower House of Japan's legislature. The DPJ's victory appears to mark 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. Although in the past some members of the DPJ have questioned and/or voted against several features of the alliance, 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."
CRS Report for Congress, RL33436