Armed conflicts between Saxons and Franks occurred repeatedly from the 6th century. It reached its peak under Charlemagne, whose expansionism focused on Saxony as well - legitimized by the simultaneous Christian missions in pagan Saxony. The Saxon Wars started in 772 with the destruction of the Saxon's most important sanctuary, Irminsul. It ended in 804, after many bloody battles and setbacks with the final conquest of Nordalbingia. Although the northeastern border of the Frankish Empire followed the Elbe, the region north of it (up to the rivers Eider and Trave), served as a buffer zone against the Vikings. Initially it was left to the allied Slavic Obodrites, but in 810 it was fully integrated into the Frankish territory. When Saxony was defeated and political integrated, a religious infrastructure was built for sustainable proselytizing of the North.