Theories of economic development suggest variously that national income increases or decreases the propensity for states to fight, while systematic evidence of the impact of development on warfare is ambiguous or non-existent. The lack of empirical support for nominally opposing claims can be reconciled if elements of both perspectives are partially correct. We use a formal model to construct an explanation linking economic development with interstate conflict that resolves contradictory theories and a relative paucity of evidence. Development increases the ability of states to project power while decreasing the willingness of states to engage in conflict over certain issues. High income states fight less often to conquer tangible assets or territory, but fight more often to compel adherence to preferred policies and to police the global commons.