 In the Middle Ages, there were three successors really of the Roman Empire. There was the Frankish Empire in the West, the empire founded by Charlemagne. There was the Caliphate which, although it defined itself against Christendom and the Romans, nevertheless was very much built on the rubble of the empire in certain ways, modelled on it. And between those two great empires was a third, and that empire really had the greatest right of all to describe itself as Roman. And that was the empire that we call Byzantium after the name of the Greek city that was refounded by Constantine the Great, the first Christian Roman emperor, as Constantinople. And although we call the peoples of that empire Byzantines, they called themselves Romans, albeit in Greek. And there is a sense in which calling them Byzantines obscures the degree to which, more than any other state in the Middle Ages, Byzantium was the Roman Empire. The Byzantines saw themselves not merely as guardians of the Roman past, but the inheritors of a glorious Christian future. Their empire, they believed, was destined to last for all time. And yet there was a problem with that, which was that it was evident that their power was not what it had once been. And so the Byzantines found themselves engaged in a constant war of attrition against the superpower to the east, the empire of the Caliphate. And the way in which the Byzantines rationalised that was to cast themselves and the Saracens as kind of twin eyes, twin poles, the great rival empires, the only equal that the Byzantines were really prepared to acknowledge. To the west, of course, they did face another empire which cast itself as Roman, and that was the empire of the Franks. Now the attitude of the Byzantines to that empire was altogether more condescending than it was towards the empire of the Saracens. Tensions between Byzantium and the Latin West existed on two levels. First of all, there was the religious dimension. Even though the Bishop of Rome, the Pope, had at one point been a subject of the Byzantine empire. The weakness of Byzantium, the failure of its attempt to keep hold of Italy meant that the Pope was obliged to look elsewhere for a patron, and he turned to the king of the Franks. And by doing that, a further dimension of rivalry was introduced, which was political. And these rivalries, both political and religious, grew increasingly poisoned over the course of the centuries that followed in particular the coronation of Charlemagne as a rival emperor in AD 800 in Rome. In the new Rome, this was regarded with a mixture of disdain and horror. In the centuries that followed that, particular rivalry was focused on that between the Pope and the patriarch of Byzantium. And in 1054, things came to a kind of head. That is the date that is conventionally given to something called the great schism, the sense that there being an irreparable rupture between the Latin and the Greek churches. That's slightly to oversimplify it, but there's no question that the increasingly uneasy relationship between the two churches that had existed throughout the early Middle Ages became altogether more poisonous in the high Middle Ages. And then a further dimension that complicated relations and indeed poisoned it was that with the launching of the Crusades, which were originally launched as a result of an appeal from the Byzantine Empire for Latin assistance against the Turks. In the wake of the Crusades, the eyes of Latin adventurers were opened up to the riches on offer within Constantinople and particularly its western provinces. And with the military strength that they brought, they also brought an increasingly lupine appetite for the splendor and the wealth that Byzantium offered. And so the paradox is that when finally the great walls of Constantinople stretching almost 12 miles, widely regarded as impregnable, when they were finally breached, they were not breached by a Muslim enemy, they were breached by the Christian knights of the Latin West when in 1204 Constantinople finally fell.