Why Diplomats Date Modern States to 1648

The Peace of Westphalia was actually two treaties signed in October 1648, in Münster and Osnabrück, ending the Thirty Years' War. The war had killed an estimated 4.5 to 8 million people across central Europe — roughly a third of the German-speaking population — over a tangle of religious and dynastic claims.

The treaties did not invent the nation-state. What they did was settle, in writing, that a ruler had final authority over religion and law within a defined territory, and that other rulers had no business intervening on behalf of co-religionists across the border. Cuius regio, eius religio, recycled from a 1555 deal but now binding on the major Catholic and Protestant powers at once.

Later scholars — Leo Gross's 1948 essay in the American Journal of International Law was particularly influential — promoted Westphalia as the founding moment of the modern international system. Critics, especially in the past three decades, have pointed out that this is a tidier story than the documents support. The treaties barely use the word sovereignty. Many of the doctrines we now associate with them developed slowly over the next two centuries.

The label has stuck anyway because diplomats need a date. When someone argues that humanitarian intervention violates national sovereignty, or that the EU pools sovereignty in unprecedented ways, the implicit baseline is Westphalian. The system being argued for or against is the one that supposedly began in 1648 — a useful fiction even when it is partly a fiction.