North Rhine–Westphalia power. The Peace of Westphalia of 1648, signed in Münster and Osnabrück, ended the Thirty Years' War. The concept of nation-state sovereignty resulting from the treaty became known as "Westphalian sovereignty". As a result of the Protestant Reformation, there is no dominant religion in Westphalia. Roman Catholicism and Lutheranism are on relatively equal footing. Lutheranism is strong in the eastern and northern parts with numerous free churches. Münster and especially Paderborn are thought of as Catholic. Osnabrück is divided almost equally between Catholicism and Protestantism. After the defeat of the Prussian Army at the Battle of Jena-Auerstedt, the Treaty of
Hessian War Peace Congress mediated by Duke Ernest of Saxe-Gotha and which resulted in a unification and peace treaty that was sealed in April 1648, ahead of the Treaty of Westphalia itself. Before the Westphalian Peace Treaty, Cassel fought one last time against the imperial side, winning the Battle of Wevelinghoven in the Rhineland, together with other Protestant troops. Darmstadt, however, was not involved in this battle. Under the Unification Treaty between Cassel and Darmstadt, Upper Hesse was permanently divided. Darmstadt had to give up a significant part of Upper Hesse to Cassel, not least Marburg and other occupied territories, including the