In 799 the new pope, Leo III, threatened with deposition by the Romans, appealed to Charlemagne. Charlemagne hastened to Rome to support Leo, and on Christmas Day, 800, was crowned emperor by the pope. His coronation legitimized Charlemagne's rule over the former Roman empire in W Europe and finalized the split between the Byzantine and Roman empires. After years of negotiation and war, Charlemagne received recognition from the Byzantine emperor Michael I in 812; in return Charlemagne renounced his claims to Istria, Venice, and Dalmatia, which he had held briefly. The end of Charlemagne's reign was troubled by the raids of Norse and Danes (see Norsemen), so Charlemagne took vigorous measures for the construction of a fleet, which his successors neglected. His land frontiers he had already protected by the creation of marches. In 813, Charlemagne designated his son Louis I as co-emperor and his successor and crowned him at Aachen.
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