Theories of international relations commonly rely upon strong assumptions about state preferences, or derive them from ancillary theories that themselves make strong assumptions about the sources of state preferences. By means of a detailed explanation of interest-formation and interest–specification in a particular case (the Japanese interest in the recovery of the Northern Territories), supplemented by comparative discussion, this paper argues that national “interests” are idiosyncratic and best treated exogenously. It assesses the implications for international relations theory, and addresses some common objections.