Pope Leo III crowned Charlemagne emperor in Rome. The coronation occurred at St. Peter’s Basilica on Christmas Day, 25 December 800, during a mass attended by Charlemagne and the Roman clergy and people.
Rome was symbolically important because it was the ancient center of Roman imperial authority and the seat of the pope. By crowning Charlemagne there, Leo connected the Frankish ruler with the legacy of the western Roman emperors, even though Charlemagne’s main political centers were north of the Alps.
The ceremony also had immediate political consequences. Constantinople, capital of the Byzantine or Eastern Roman Empire, viewed itself as the legitimate Roman imperial center. The appearance of a new emperor in the West therefore created diplomatic friction, although the Byzantine government was not in a strong position to challenge Charlemagne militarily at the time.
Aachen was Charlemagne’s principal residence and later became closely associated with his imperial memory, but it was not the site of his coronation. Paris and Milan likewise played important roles in later French and Italian history, not this ceremony.