The negotiations that ended the Thirty Years' War were the first great multilateral congress in European diplomatic history. Talks ran for five years in two cities of the Westphalia region: Catholic envoys convened at Münster, Protestant envoys at Osnabrück; couriers carried decisions between them. Emperor Ferdinand III, the kingdoms of France and Sweden, the Dutch Republic, Spain, and the hundreds of principalities and free cities of the Holy Roman Empire were represented. On 24 October 1648 the treaties of Münster and Osnabrück were signed.

The concrete terms answered the destruction. Calvinism was added as a legitimate confession alongside the Catholicism and Lutheranism recognised in the 1555 Peace of Augsburg. The principalities within the Holy Roman Empire received broad autonomy — including the right to conduct foreign policy and form alliances — which in practice turned the Empire into a confederation of more than three hundred semi-sovereign units. The independence of the Dutch Republic and the Swiss Confederation was formally recognised. France made significant gains in Alsace; Sweden obtained strategic territory along the Baltic coast. The papacy rejected the treaties — a clear emblem of its loss of authority over European politics.

The idea that the treaties 'gave birth to the modern state system' is itself a reading constructed by 19th- and 20th-century international relations theorists (most influentially Leo Gross's 1948 article). In the 1648 texts the word 'sovereignty' is still embedded in feudal legal language; non-interference is expressed not as a modern absolute principle but as an internal arrangement of the Holy Roman Empire. Historians such as Andreas Osiander and Stephen Krasner have shown that the 'Westphalian system' narrative is less a description of the treaties than a founding myth that international law needed in the centuries that followed. Yet the practical effect of the treaties — the withdrawal of the papacy's universal claim, the mutual recognition of states as legal equals, the displacement of religion from the center of diplomacy — did fix the direction of modern European politics.

After 1648 Europe became a continent organised not around a shared religious truth but around balanced relations among limited sovereign states. Permanent embassies, multilateral congresses, state archives, modern diplomatic protocol — all were built in the century that followed. Even if Westphalia did not, as is often said, 'invent modern sovereignty', it remains the symbolic anchor of a three-century European order — and, indirectly, of a non-interference norm that runs all the way to the UN Charter. In the 19th century Westphalia became a date; in the 20th century a global myth; in the 21st century it is still cited as the reference point in debates over transnational sovereignty.