Building on a careful analysis of Southeast Asia’s recent history, politics, economics, and place within the Asia Pacific, this report looks forward two decades to anticipate the development of trends in the region and how they will impact the U.S.-Japan alliance. How will Southeast Asian states come to grips with the political and economic rise of China? How will they modernize their military forces and security relationships, and what role can the United States and Japan play? How will they manage their disputes in the South China Sea, and how will they pursue greater regional integration? These questions will prove critical in understanding Southeast Asia’s role in the Asia Pacific, and in the U.S.-Japan alliance, in the decades ahead.