From the Overview: "Japan, a U.S. treaty ally since 1951, is a significant partner of the United States in several foreign policy areas, particularly security and trade issues. Shared security goals range from meeting the challenge of an increasingly powerful China to countering threats from North Korea. The mutual defense treaty grants the U.S. military the right to base U.S. troops--currently numbering around 50,000-- and other military assets on Japanese territory in return for a U.S. pledge to protect Japan's security. The two countries collaborate across multiple bilateral, regional, and multilateral institutions on issues such as science and technology, global health, energy, and agriculture. Japan also is the fifth largest U.S. trading partner (fourth largest if European Union [EU] countries are considered separately) and second largest source of foreign direct investment in the United States. Japanese investors are the largest foreign holder of U.S. Treasury securities. Despite their shared security concerns and the personal rapport that has developed between Prime Minister Shinzo Abe and President Donald Trump, significant policy differences between Washington and Tokyo have emerged."
CRS In Focus, IF10199
Congressional Research Service: https://crsreports.congress.gov/