Among the Anglo-American principles of war, unity of command is nearly paramount. Yet, in interagency operations and coalition warfare, unity of command often is impossible to achieve. As a result, unity of effort may well be the only realistic goal. This chapter examines the challenge of achieving unity of effort as a part of political-military theory that is at once empirical and normative. Based on classical writing and observed recent events (the latter serving the same purpose they did for the classical theorists), the chapter focuses on three specific problems. First, it addresses multiservice, or joint, issues; second, it considers those of the civil-military arena; lastly, it looks at multinational challenges in settings of mature alliances and ad hoc coalitions.