On Christmas Day, 800 AD, Pope Leo III placed a crown on the head of the Frankish king Charles in St. Peter’s Basilica in Rome and the assembled congregation acclaimed him Emperor of the Romans. Whether Charlemagne was surprised by this — his biographer Einhard claims he said he would not have entered the church had he known what was to happen — is debated; the staging suggests coordination, and Charlemagne was not a man who was often genuinely surprised by political events. What is not debated is what the coronation meant: four centuries after the conventional date of Rome’s fall, the most powerful ruler in western Europe was being crowned not as King of the Franks or King of the Germans but as Emperor of the Romans, in Rome, at the greatest shrine of Roman Christianity, by the successor of St. Peter. Rome had not died. It had changed form.