A historical region and former duchy of west-central Germany east of the Rhine River. The duchy was created in the 12th century and was administered for many centuries by ecclesiastical princes, especially the archbishop of Cologne. The Peace of Westphalia (1648) marked the end of the Thirty Years' War. Napoleon seized the area in 1807 and designated a portion of it as the kingdom of Westphalia, to be ruled by his brother Jérôme. The region became part of Prussia after 1815.