International Law and the Limits of State Sovereignty

The modern international legal order is conventionally dated to the Peace of Westphalia (1648), the treaties ending the Thirty Years' War, which is credited — with important qualifications from historians who note that fully sovereign, mutually recognizing territorial states developed only gradually over the following centuries — with establishing the principle of state sovereignty: that each state exercises supreme, exclusive authority within its own territory, free from external interference in its internal affairs, and that states are formally equal in international law regardless of size or power. This "Westphalian" picture underlies the traditional view of international law as fundamentally consensual, resting on state consent expressed through treaties (conventional international law) and customary practice accepted as legally binding (opinio juris), a positivist conception given influential expression by legal theorists such as Hans Kelsen, who nonetheless sought, in works including Peace Through Law (1944), to place international law within a single hierarchical legal order (the Grundnorm framework of his Pure Theory of Law) rather than treating it as a fundamentally separate normative system from domestic law.