Western governments have increasingly adopted comprehensive approaches to overseas missions, involving civilian, political and military elements. In Afghanistan, comprehensive policies introduced by national governments created new imperatives for unity of effort between government departments/agencies and armed forces. Using the lens of the historical institutional theory of gradual change, this research examines the American, British and Canadian militaries’ adaptation to unity of effort requirements identified by governments for missions in Afghanistan in the period 2001 to 2011. Each case examines how the armed forces’ existing institutional rules, forged from past experiences and operations in Bosnia-Herzegovina, and the domestic context of national missions, led to specific changes in formal doctrine, in-country command arrangements, and Civil-Military Cooperation capabilities. The study identifies institutional and political factors that influence militaries’ role in the delivery of the comprehensive approach. It finds that armed forces’ contribution to the approach depends on: the level of discretion afforded institutional leaders to interpret rules guiding interagency efforts; and the ability of national governments to create a formal civilian entity to coordinate and deploy qualified personnel to work alongside armed forces overseas. Understanding these factors is key to further developing military capacities to work with civilian entities in future overseas missions.