Why was a Frankish king crowned emperor on Christmas Day 800?

A pope in trouble needed a protector, and the strongest king in Europe was the obvious choice. On Christmas Day 800, at mass in St. Peter's in Rome, Pope Leo III crowned Charlemagne "emperor of the Romans". The Frankish king ruled most of Western and Central Europe, and no emperor had reigned in the west since 476. Historians still argue about how much of the scene Charlemagne planned, but the pope's need and the king's power explain why it happened.

The story starts with a crisis. Charlemagne had already become the chief defender of the papacy, and in 774 he had removed the Lombards from power in northern Italy. Then in April 799 Leo's enemies attacked him, and the pope fled north to ask Charlemagne for help. The coronation came the following year.

Was it a plan or a surprise? Charlemagne's biographer Einhard says the king would never have entered the church had he known what the pope intended. Some historians take this at face value, and others think it was a literary way of showing humility. Whatever his intention, the title had consequences: it began a centuries-long quarrel with Constantinople, known as the problem of two emperors.

Charlemagne was no scholar king in the modern sense. Nobody is sure he could read: he had texts read aloud to him and dictated his decrees, though that was normal even for literate rulers, and Einhard says he tried to learn to write only later in life. Yet he promoted schools where students learned Latin, grammar and arithmetic, and his scribes copied ancient authors: some of the earliest surviving copies of Roman writers come from his empire.