When in 936 King Otto I had ascended the throne, he appointed Hermann a margrave ("princeps militiae"), granting him the Saxon march north of the Elbe river. His Billung March stretched from the "Limes Saxoniae" in the west along the Baltic coast to the Peene River in the east, roughly corresponding with the later Mecklenburg region. Otto thereby disregarded the claims of Hermann's elder brother Count Wichmann, a brother-in-law of Queen Dowager Matilda. Wichmann in turn joined the unsuccessful rebellion of King Otto's half-brother Thankmar and Duke Eberhard of Franconia in 938. Having more autonomy than the contemporary margrave Gero ruling over the adjacent "Marca Geronis" in the south, Hermann exacted tribute from the local Polabian Slavs of the Obotrite tribal federation.