The regions most at risk from climate change are also those regions with the highest incidents of interstate conflict, radicalism, and terrorism. These regions include South Asia, North Africa, Sub-Saharan Africa, and Central America. Climate change will further exacerbate regional tensions, increasing the risk of conflict, mass migration, and humanitarian emergencies that will jeopardize U.S. national interests and — if history is a guide — force U.S. intervention. Even if climate change does not result in more numerous American military operations, it will affect the stability and capabilities of numerous friends and allies. Climate change will, in a fundamental sense, be a key element in defining threats to American national security in the future.