Presents a theory of domestic politics, showing how it affects international outcomes. In developing this rational choice theory, the text argues that any explanation that treats states as unitary actors is misleading. It describes all states as polyarchic, where decision-making power is shared between two or more actors (such as legislature and executive). A model is constructed based on two-level game theory, reflecting the political activity at both the domestic and international levels. The model is illustrated by the question of co-operation between nations.