This report examines some of the most significant factors shaping the future of warfare over the next ten to15 years changes in the size, quality, and character of military forces available to the United States and its potential adversaries. In it, we follow sponsor guidance to focus on overarching trends rather than to provide an adversary-by adversary analysis, and we avoid trends covered in other volumes of this series. Instead, this volume identifies six significant trends that will shape who and where the United States is most likely to fight in the future, how those wars will be conducted, and why they will occur. Table S.1 summarizes findings regarding these trends and the challenges they present. As the table indicates, the risks of war over the next ten to 15 years will derive largely from perceptions of shifts in regional correlations of force. With U.S. conventional forces reduced in size, Chinaand, to a lesser extent, Russiawill narrow the qualitative gap and might calculate that the United States lacks sufficient capacity to respond effectively. China and Russia, however, likely will prefer to achieve their objectives on the cheapi.e., with the least cost in international reproach and the lowest risk of provoking military conflict with the United States. Instead, both likely will ramp up their use of gray-zone tacticsemploying incremental aggression, information warfare, proxy forces, and covert special operations forces to obtain their regional objectives but staying below the U.S. threshold of conventional response.