Sachsenkriege Karls des Großen
The northern border of Holstein along the Eider River had already formed the northern border of the Carolingian Empire, after Emperor Charlemagne upon the Saxon Wars reached an agreement with King Hemming of Denmark in 811. The lands of Schleswig beyond the river remained a fief of the Danish Crown, while Holstein became an integral part of East Francia, the Kingdom of Germany and the Holy Roman Empire.