The dramatic disappearance of the Roman empire in the West left a great void: the political home in which millions of people had lived for centuries was no more. Finally it was unable to survive the twofold onslaught of the Germanic peoples in the north and the Arabs in the south. First the Pars Occidentis, with Rome herself, fell to the tribes from beyond the Rhine–Danube limes; afterwards much of the Pars Orientis, Byzantine ‘new Rome’, fell to Islam. From the storms that raged from the fifth to the seventh century three worlds emerged, three heirs of classical Antiquity. Greek Christendom in Byzantium was the direct, but much reduced continuation of the Roman empire in the east, ruled by a Roman emperor and living under Roman law. Latin Christendom lived under Germanic tribal kings, who had taken their religion from Rome, but little culture and even less law. The Arab world of Islam, stretching from Asia via northern Africa to Spain, had inherited not much law and even less religion from Antiquity, but a good deal of Greek science.

It is in the Latin Christian world of the early Middle Ages, from the fifth century onwards, that our narrative starts. The void which originated there was caused by the loss of some fundamental ideas and structures. The first victim was the centralized imperial state itself and with it the basic notion that its millions of multiracial inhabitants were subjected to one common domination, and that no other autonomous kernels of legitimate power existed beside it: all public authority had been vested in the emperor or descended from him.