Westphalia first appears as the name of the western third of the duchy of Saxony in the 10th cent. Unlike Eastphalia, the eastern third of the duchy of Saxony, Westphalia survived the breakup (1180) of the Saxon duchy as a regional concept, although it lost political unity. The larger part of Westphalia came under the rule of ecclesiastical princes—the bishops of Münster, Osnabrück, Minden, and Paderborn and the archbishops of Cologne, who obtained the region around Arnsberg, known as the duchy of Westphalia. Among the temporal fiefs that emerged from the breakup of Saxony were the counties of Lippe, Ravensberg, and Mark. All these territories were later included in the Westphalian Circle of the Holy Roman Empire (formed c.1500), which also encompassed considerable non-Westphalian land. In the later Middle Ages most of the important Westphalian towns—e.g., Münster, Osnabrück, Paderborn, Bielefeld, and Soest—prospered as members of the Hanseatic League.
The bishoprics of Münster, Paderborn, and Osnabrück and the duchy of Westphalia were secularized only in 1803 by the Diet of Regensburg as a result of the French Revolutionary wars; they were at first partitioned among Prussia, Hanover, Hesse-Darmstadt, Hesse-Kassel, and the grand duchy of Berg. In 1807, after the signing of the Treaty of Tilsit (see Sovetsk), Napoleon seized all Prussian possessions W of the Elbe, as well as the electorates of Hesse-Kassel and Hanover and the duchy of Brunswick. The northern section of these territories, including Münster, was directly annexed by France. The southern section was constituted as the kingdom of Westphalia, with Napoleon's brother Jérôme Bonaparte (see Bonaparte, family) as king and with Kassel as the capital. The kingdom, which actually included only a small part of Westphalia, collapsed in 1813. At the Congress of Vienna the major part of Westphalia proper was awarded (1815) to Prussia; and Hanover, Hesse-Kassel, and Brunswick were restored. Westphalia continued as a Prussian province until 1945.