The United States Department of Defense and U.S. Agency for International Development have interacted for 50 years to advance national security interests. With origins in the Marshall Plan, and through joint efforts in the Vietnam, Iraq and Afghanistan wars, the two have developed policies, liaison systems, and joint programming to advance practical coordination. After closely-combined defense, diplomatic and developmental (3D) efforts, USAID and DOD have never appreciated each other's capabilities better. Despite this, significant challenges exist which impede sustained coordination, including resource imbalances, conceptual gaps, and personality-based rather than institutional relationships. As war efforts conclude, is a window of time closing on dev-mil coordination? What are the implications for unity of effort between military and development actors? This book analyses the history, policies, coordination structures, and experiences of USAID and DOD interaction; identifies trends and challenges; and recommends continued interagency engagement, particularly through joint planning, field programming and broader staff exchanges.