Scholars of international relations generate a wide range of theories to solve the problems and puzzles of state behavior. Each theory offers a causal account of a particular outcome or pattern of behavior in interstate relations in a form that isolates independent and dependent variables precisely enough to generate hypotheses (predictions) that can be empirically tested. At a higher level of generality, these theories can be grouped into different families or approaches on the basis of their underlying analytical assumptions about the nature of states and the relative explanatory power of broad classes of causal factors, such as the distribution of power in the international system, international institutions, national ideology and domestic political structure.

This chapter will summarize the three main theoretical approaches used in contemporary American political science: Realism, Institutionalism and Liberalism. Political scientists are likely to find the versions presented here overly simplified and distilled. Yet each approach gives rise to a distinct mental map of the international system, specifying the principal actors within it, the forces driving or motivating those actors and the constraints imposed on those actors by the nature of the system itself. Anyone who thinks about foreign policy or international relations, from either a political or a legal standpoint, must have some such map to guide her thinking, whether consciously or subconsciously.

Beyond mental geography, however, the explicit role of theory differs for political scientists and lawyers.