One 1648 peace deal quietly invented the idea of a sovereign nation-state

The Peace of Westphalia ended the Thirty Years' War, a conflict that had killed an estimated 4.5 to 8 million people across Central Europe. Its treaties established that each state held supreme authority over its own territory, free from outside interference — a principle that became the backbone of the modern international system of nation-states. Historians now debate how sharp a break Westphalia really was, since elements of state sovereignty existed both before and long after 1648.

— Standard historical accounts, Peace of Westphalia — Osnabrück and Münster, 1648