Dithmarschen (dĭtˈmärshən) [key], region, SW Schleswig-Holstein, N Germany, between the Elbe and Eider rivers. It is chiefly an agricultural region, with extensive cattle raising in the west. The eastern portion is a sandy upland. There are oil fields near Heide. The region was conquered by Charlemagne, and its population was Christianized. Later in the Middle Ages it became a virtually independent peasant republic. In 1474, Emperor Frederick III incorporated Dithmarschen into Holstein and invested Christian I of Denmark with the fief, but the attempts of the Danish kings and nobles to take possession of the region were repulsed by the peasants until 1559. Dithmarschen passed to Prussia in 1867. The region is also called Ditmarsh.