The United States faces a continuing need to conduct interagency operations, especially between the military and USAID. Surprisingly, however, this field has been treated much too casually in light of its merits as a potential force multiplier-and as a source of serious operational problems. US leaders will continue to choose the interagency approach (over unilateral options) to solve the sorts of complex problems that demand action and systematic intervention, yet allow room for political maneuver. By studying the activities of two inherently adversarial bureaucracies (USAID and the USMWGP) which needed to work together to solve a complex counterinsurgency problem in El Salvador, I have been able to determine which factors are most important for unity of effort in future interagency operations. In El Salvador the mission was to combat insurgents, but the principles and many of the tasks required to succeed then and there, apply today and elsewhere in waging the wars against drugs, terror, and other post- Cold War security threats.