The Ottonian dynasty supported eastward expansion of the Holy Roman Empire towards Wendish (West Slavic) lands during the 10th century. The campaigns of King Henry the Fowler and Emperor Otto the Great led to the introduction of burgwards to protect German conquests in the lands of the Sorbs. Otto's lieutenants, Margraves Gero and Hermann Billung, advanced eastward and northward respectively to claim tribute from conquered Slavs. Bishoprics were established at Meissen, Brandenburg, Havelberg, and Oldenburg to administer the territory. A majority of Wendish tribes had been Christianized from the German conquests, but in 983 they returned to paganism when a great Slavic rebellion reversed the initial German gains. While the burgwards allowed the Saxons to retain control of Meissen, they lost Brandenburg and Havelberg. The Elbe River then became the eastern limit of German-Roman control.