Westphalia to be Catholic. Osnabrück was divided almost equally between Catholicism and Protestantism. Parts of Westphalia came under Brandenburg-Prussian control during the 17th and 18th centuries, but most of it remained divided by duchies and other areas of feudal 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". After the defeat of the Prussian Army by the French at the Battle of Jena–Auerstedt, the Treaty of Tilsit in 1807 made the easternmost portion of today's Westphalia part of the
Treaty of Altranstädt (1707) Treaty of Altranstädt (1707) The Treaty or Convention of Altranstädt was signed between Charles XII of Sweden and Joseph I, Holy Roman Emperor on 31 August 1707. It settled the rights of Protestants in Silesia. While the Protestant Reformation had strongly affected Silesia, the Habsburg emperors had subjected the province to the counter-reformation in the 18th century. Especially in Upper Silesia, these measures were successful: in the early 18th century, almost half of the Silesian population was Roman Catholic and some 1,000 churches had been rededicated from Protestant to Roman Catholic. The Peace of Westphalia (1648) protected Protestants only in