From the Document: "Japan is a significant partner of the United States in a number of foreign policy areas, including addressing regional security concerns, which range from hedging against Chinese military modernization to countering threats from North Korea. The U.S.-Japan military alliance, formed in 1952, grants the U.S. military the right to base U.S. troops--currently around 54,000 strong-- and other military assets on Japanese territory, undergirding the 'forward deployment' of U.S. troops in East Asia. In return, the United States pledges to help defend Japan. The two countries collaborate through multiple bilateral and multilateral institutions on issues such as science and technology, global health, energy, and agriculture. [...] This report contains two main parts: a section describing recent events and a longer background section on key elements of the U.S.-Japan relationship."
CRS Report for Congress, RL33436
Congressional Research Service: https://crsreports.congress.gov/