In a period of budget austerity and emerging threats, defense policymakers must contend with a range of competing requirements. And while current conflicts take precedence, the Defense Department must also focus on developing the force of the future. To do so, it is first necessary to consider the nature of conflict and types of missions the U.S. military may face in the coming decades. 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 and examines the drivers of the FSE in order to guide analysis and decisionmaking.