International law comprises a system of norms, customs, rules, treaties, and agreements that purports to govern the behavior of nation states toward their citizens, other states, and other states' citizens. The very breadth of international law frustrates categorization. At some level of abstraction, any generalizations about the purposes and functions of international law cannot avoid gross oversimplification. But some themes run through the field, even if they do not explain every aspect of the system.

Although nations may conform their behavior to international rules and norms for many reasons, the animating purpose of much international law is to foster mutually beneficial cooperation among states. Although the subject matter of any particular cooperative enterprise may vary greatly – from treaties and customs regarding a private investment agreement to a convention on global warming to the “laws” of war – the fundamental behavior remains the same. International law functions in many instances to encourage nations to cooperate where it is in their mutual self-interest to secure the cooperation of other states, but where deviations from the cooperative goal will secure even greater private advantage to any single defecting state. To solve this conundrum, a primary concern of any system of international law is insuring compliance with the cooperative goal.