On Christmas Day 800 Charlemagne, King of the Franks, was crowned by Pope Leo III in the Basilica of St Peter in Rome and became "Imperator Romanum Gubernans Imperium".
A new Christian Roman Empire of the West had been born, and at its head stood a man who was to be claimed as a symbolic ancestor by French monarchs, Holy Roman emperors and German kaisers. France and Germany owe their shape to the division upon his death of the empire into West and East Francia, while the European Union itself can be considered, as Hywel Williams points out in his scholarly and engrossing new book, as "western Europe recovering its Carolingian origins".
Even the relative isolation of Britain and Scandinavia from the core European project echoes those countries' relationship with Charlemagne's dominions; not formally under their powerful continental neighbour's sway, yet sharing borders and being influenced by it nevertheless.
With Charlemagne begins the history of modern Europe. This is not a matter of strict territorial boundaries, since much of what makes up the geographical continent was not in his hands. But Europe as "a certain idea", to apply the words of one of Charlemagne's spiritual descendants, France's General de Gaulle (who, like the emperor, was also both long of nose and in stature), does trace its origin to this warrior king who subdued Saxons and Lombards, nomadic Avars and rebellious Thuringians; but who also brought them under his law and one church, and who instigated a cultural and linguistic renaissance from which a unity flowed even to those lands beyond his realm.
If there was to be a new Roman Empire, however, it need not have been a Frankish one. It could just as well have been created by the Visigoths, who had unified most of the Iberian Peninsula in the 6th century and whose Romanised civilisation was far more sophisticated than that of the Franks. Even the Vandals might plausibly have had a go. History has damned them as perpetrators of "senseless destruction" but, as Williams describes in one of the many delightful diversions strewn through his text, it was their successful blockade of grain from North Africa that earned them the Romans' bitterness and their subsequent dire reputation. In fact, once settled around Carthage and Hippo they "adopted the creature comforts of the late Roman aristocracy with some aplomb".
No new empire was forged by any of the Germanic tribes who had settled around the Mediterranean, though. The last Imperator of the West, Julius Nepos, had died in 480, and apart from a brief period when the Emperor Justinian reconquered many of the old provinces, neither could the Byzantines revive the Roman state to which they were the eastern successors. Some of this was down to the divisions and shifting alliances that sapped the strength of many of the actors. But by the time Charlemagne was born a new power had risen in Asia. And one of the most fascinating themes that runs through this wide-ranging account of his life and dynasty is this: to what extent did the Europe to which Charlemagne was father only come into being because of the birth of Islam?
For centuries, civilisation had flourished by the Mediterranean Sea and the Fertile Crescent. Great cities and trading empires were built when the north was still stuck in a damp and primitive Iron Age, the speech of its inhabitants so incomprehensible, according to one theory, that they were named barbarians as their vocabulary seemed to consist of no more than variants on "bar-bar". Francia was not the most obvious candidate to produce a new Constantine. Shortly after the Prophet Muhammad's death in 632, however, the armies of a new, monotheistic faith began to emerge from Arabia and carry all before them. Persian and Byzantine lands were the first to fall, and by 711 Muslim forces had reached Spain, vanquishing the Visigoths within a few years.
By the time of Charlemagne, the major cities of Provence had been occupied or destroyed, the eastern markets were no longer freely available to the kingdoms of the west, and "the Mediterranean looked as if it might become a Muslim lake". To the Christians these were enemies, and they were at the gates. The victory of Charlemagne's grandfather Charles Martel over the invaders at the Battle of Tours in 732 may well have been no more than a "skirmish", but it has become as totemic as the failure of the Siege of Vienna in 1683 in the story of how the Muslim advance on Europe was halted. And for the nascent Carolingian dynasty (Martel was technically only "Mayor of the Palace" to the reigning Merovingians, who were formally displaced from the throne by his son Pippin III), it was "the supreme moment of Frankish opportunity". None had been able to hold fast in the face of the Arab armies - until the new rulers of the Franks.
Charlemagne's martial endeavours focused mainly on the east - his attempts on Spain were limited - and his relations with the Abbasid court in Damascus were cordial.
Still, it is not fanciful to date that European antipathy to Islam, which has found expression recently in EU reluctance to implement Turkish accession to the Carolingian clash with the new world power.
For these were years when identities that would later crystallise as a Christendom of European peoples were forming. As Williams writes: "War and conquest pushed them to new frontiers where they encountered non-Christians. Confronted with cultures that were entirely alien to them, the Franks gained a keener appreciation of their own difference and came to exult in that distinctiveness ... Nations need their enemies, and national identities are built on the graves of foes."
But propitious circumstances alone do not explain why the name of Carolus would forever after be followed by the suffix magnus. In this revised edition, Williams elucidates how Charlemagne drew on prior pagan notions of Germanic sacral kingship, fused them with the Roman culture of the lands the tribes now occupied, and sealed this potent admixture with the papal blessing: and this last was crucial. Surrounded by semi-hostile states and barely in control of his own city, the Bishop of Rome needed a protector. Charlemagne fitted the bill. In return for safeguarding the pope and his authority, the king became the first emperor of the West for 300 years, and one now recognised both spiritually and temporally. The link between the cross and the sword was enthusiastically put into practice on the battlefields of the east, whose populations were forced to convert. (That is when they weren't beheaded, as 4,500 Saxons were at Verden in 782, an atrocity that the Saxon SS chief Heinrich Himmler was later to commemorate.) European monarchs would define themselves as soldiers of Christ for a millennium to come.
Warfare, in which Charlemagne excelled, provided the spoils to keep the Frankish nobility content. But his kingship had greater purpose. Previous conquerors merely plundered and claimed tribute from defeated tribes, but Charlemagne incorporated new territories into his realm and extended to them a protection under the law that was highly advanced for the time.
It was this imposition of unity over his empire and the formalising of language and religion - Charlemagne was severe on the adoptionist and Arian heresies, which contradicted the doctrine of the Trinity, and codified the use of Latin and the Roman liturgy - that made his legacy so long lasting. Hereditary officeholding, of the position of count, for instance, and the formal organisation of the royal courts that were such a feature of medieval monarchy, both date to his reign; as did the script used in the first printing presses that we know today as "Venetian".
All of this began to lead to the development of notions of coherent nationality, even if they were, argues Williams, essentially inventions. "The idea that peoples had core identities whose collective origins were lost in the mists of time, or at any rate perhaps buried within the undergrowth of ancestral German forests, acquired a great vogue. This might have been nonsense, but it was historically significant nonsense because so many subjects of so many kings started to believe that it was true."
Europe as Christendom was the great endeavour of Charlemagne. And although some militant secularists like to deny it, its imprint is still visible. If anyone should doubt that, they need only consider the confused and inconsistent attitudes the continent's leaders have displayed towards the countries of the Arab Spring. Most were once ruled by Rome. Yet that is not history enough for their peoples to be considered brothers. The ghost of Charlemagne still wanders the West, whispering to its citizens to maintain the barricades of the empire he created.
Sholto Byrnes is a contributing editor of The New Statesman.