In a period of budget austerity, prioritization becomes especially important for defense policymakers. In order to determine such priorities, it is first necessary to consider the nature of conflict and missions the U.S. military may face in the future. By assessing the key components, or drivers, of the future security environment (FSE), an unknowable future becomes a bit clearer. This report provides such an assessment. Drawing on qualitative data such as national security and foreign policy literature, Defense Department strategy and operational documents, and interviews with leading academics and practitioners, this study identifies the drivers of the FSE in order to guide analysis and decision making.