Explores the concept of 'background' taken-for-granted ideas in politics, linking this notion to Habermas' 'lifeworld' and Focault's 'episteme' • Explicates decision-makers' background ideas about the utility and effectiveness of war • Suggests that offensive war is not as effective as decision-makers tend to assume and that decision-makers tend to discount the costs of war • Shows that beliefs about the effectiveness of war-specifically the articulation of military necessity and proportionality-tend to weaken the just war and international law limitations on war 'Background ideas' are the essential foundation for decision-making, action, and institutionalized practices. These ideas are sometimes explicitly articulated-when defended or asserted in new contexts-but more often simply assumed and unstated. Decision-makers often believe that war works-that it is effective-and moreover that war can achieve objectives at comparatively low cost. Jus ad bellum and jus in bello considerations in the just war tradition-including last resort, necessity, probability of success, and proportionality, and the idea of double effect-are also explicitly concerned with utility-whether and when war works. Utility is also an essential element in the discourse about military necessity. Does war work? What is it good for? Why does it tend to extremes of violence? What are its costs? How do decision-makers hold onto beliefs about the utility of war in the face of disconfirming evidence?