Charlemagne and his descendants were not able to sustain empire. There were many physical reasons for this – the difficulty of defending vast reaches, invasions by Vikings, Saracens, and Hungarians, etc.
But the root of the demise was an idea – an idea so strongly held that it overcame the most powerful entity in Western Europe since the fall of Rome. It was the idea of private law.
The Carolingians tried to bury this idea; it was not conducive to empire. In order to sustain empire, the emperor required the consent of his subjects:
…the ruler who devised a scheme of defence should also be capable of sustaining the morale of his men.
The Carolingians had ever larger borders to defend. They required new and ever-increasing support from the various kings and lords to make this defense effective. These kings and lords had to want to support the emperor. But why did their opinion matter? Couldn’t the emperor just pass a law, force compliance?
A king could only command his subjects to perform those services to which they had already been liable, and a lord could only expect customary services from his vassals. He had no right to invent new duties, and his subjects or vassals would not perform them, unless they had voluntarily given their consent.
Law by consent, not by compulsion. (continue reading)