The United States must be prepared to operate in a range of complex environments to meet a range of security challenges and threats, such as humanitarian emergencies, terrorism and violent extremism, great power aggression, health security crises, and international criminal violence. This study focuses on these five functional security imperatives and illustrates each imperative through regionally or subnationally defined operating environments. In each case, the selected security imperative must be addressed in the near term to help meet other U.S. objectives. The goal of the case studies is to characterize the operating environment, identify key tasks and responsibilities to address the security imperative, and develop a set of tools and policy recommendations for operating in those specific environments.