Treaty of Westphalia (1648)

The pair of peace treaties — the Peace of Osnabrück and the Peace of Münster — that ended the Thirty Years' War and laid the foundations of the modern international order. Westphalia established the principle of state sovereignty: that each political entity has supreme authority within its own territory and that external powers have no right to interfere in its internal affairs. The system it created shaped international law and diplomacy for the next four centuries, and remains the baseline framework from which alternatives to the nation-state must depart.