The great expansion of Islam had begun within a generation of the death of Muhammad. It became necessary, for the first caliphs, like Roman generals, quickly came to depend on plunder to survive. 'It was conquer or perish,' Sihtric said. So the armies of Damascus exploded out of Arabia, swept west across North Africa, and stormed across the Pillars of Hercules and through the Gothic kingdom of Spain.

And, with al-Andalus under the control of governors appointed by the Damascus caliphate, the raiding armies went further north still. The Muslim armies crossed the Pyrenees to attack Septimania, a Gothic domain within Gaul. They were Arabs and Berbers, men of the east and of Africa, now pouring into the green belly of Gaul.

Soon, under an able general called Abd al-Rahman, all the cities of the Mediterranean coast of Gaul were in Muslim hands. It was the eighth century. Less than a decade since the first crossing of the strait.

'There were fault lines, on both the Moorish and Christian sides,' Sihtric said. 'Abd al-Rahman always had trouble with the Berbers. One Berber general called Munuza managed to carve out an independent kingdom for himself in northern Spain, bordering Gaul. And his neighbour in Gaul was the Duke Odo of Aquitaine, who had nominally pledged allegiance to the French kings, but like Munuza craved his own independence. Both pebbles in the shoes of their respective rulers, you see.

'Well, Abd al-Rahman had a tidy mind. He shook out both these pebbles. He killed Munuza, and then crossed the mountains into Aquitaine. Odo's forces were defeated, and Abd al-Rahman, leading his army in person, drove forward, thrusting deep into Gaul. Fifteen thousand men he had, to carry out the usual burning, looting, massacring, enslavement and so forth. And he advanced to within two hundred miles of Paris, to a place called Poitiers.'

'I know it. Not far from the sanctuary of Saint Martin of Tours.'

'And there, history turned,' Sihtric said. 'The Muslims were at the door to the "Great Land", as they called it, of western Europe. Perhaps they could go further – perhaps they could advance all the way to Constantinople.

'But there, on the Roman road north of Poitiers, al-Rahman faced the army of the Frankish king, the last significant obstacle between the Muslims and all Europe.'

Orm knew the story. 'Charles Martel. The Hammer.'

'Well, Charles became known as "the Hammer" only after his great victory, after he saved Europe for Christendom. A story told to every young Christian warrior since! But it need not have been so. This is where we come to the rent in time's tapestry, Orm. This is what happened…'

Odo of Aquitaine, his army defeated by the Moors, his cities stormed, his people slaughtered or enslaved, was in despair. His only possible ally was Charles of the Franks, on whom Odo had previously made war himself. Odo considered surrendering to the Muslims, who might prove more merciful to him and his family than the Christians.

Sihtric said, 'He seems to have been a poor sort of a man, and a worse ruler. But then he got help. A monk turned up at Odo's camp. He rode only a humble ass, like Christ entering Jerusalem, and carried nothing, no food, not even a bottle of water. He relied for his life on the Christian charity of the folk whose lands he crossed. He was a peculiar type, too well-fed to be a monk – and well-spoken, with a peculiarly accented Latin on his lips. He impressed with minor miracles – fortune-telling, an ability to predict bad storms and harsh winters, that sort of thing. He said his name was Alfred, he was from the famous monastery of Lindisfarne in England, and he had a message from Christ for Odo.'

'He was al-Hafredi.'

'Well yes, but don't run ahead of the story, Viking. He might have been killed, for Odo's troops were by this time cowering from shadows. He did have his wretched ass stolen, slaughtered and eaten by the starving warriors. But he was let live, and was admitted to Odo's presence. And there he changed Odo's mind.'

'How?' Orm asked.

Sihtric shrugged. 'I don't know what he said to Odo. I don't know what he promised him, what he showed him. But, Viking, if I knew your future, all of it, it would not be hard to manipulate you. Perhaps you can see that.

'In any event al-Hafredi persuaded Odo that he should not submit to the Moors, and despite his hostility to Charles he should throw himself on the Frank's mercy and face the Moors with him. And he whispered to Odo how the Moors might be beaten.'

'And Odo was convinced by this?'

'He must have been. For that's precisely what he did.'

So the Moors faced the Franks, that October three centuries past, the future of all Europe at stake. The two forces had been well matched. Charles was a proven war leader, as was Abd al-Rahman. There followed seven days of inconclusive skirmishes and scouting.

At last the combat came. Odo's weary forces made little difference to Charles's military strength – but the advice Odo was able to give on how the Moors fought, and how Abd al-Rahman thought, was much more valuable.

'The Moorish cavalry charged. But Charles's infantry held their ground. The Muslims were taken aback. They had been used to Christians breaking and running from their advances. In that one moment the battle turned, just as al-Hafredi said it would. And then Charles shocked the Moors by attacking them aggressively.

'In the combat that followed, Abd al-Rahman was killed. I have always wondered if al-Hafredi had something to do with it – perhaps that drab monkish habit concealed a knife. The Moors were not broken. They could have fought on, but without their leader, they chose to turn away.

'The battle itself was inconclusive. But it was a crucial day, in all our histories – Bede of Jarrow knew this, in faraway England. The Moors had come a thousand miles north from the strait to Africa. Now at last they had been turned back. And though they continued to raid southern Gaul, they were never to progress so far north again. Why, less than a century later Charles's grandson Charlemagne was mounting expeditions the other way across the Pyrenees.'

'Christendom was saved, then. And what became of al-Hafredi? How did his story come down to you?'

'He was wounded in the battle, it's said, an arrow in the back, but it did not penetrate deep enough to kill him. He survived, and, thanks to a grateful Odo, he was feted as something of a hero. He ended his life in Spain, in the city of Santiago de Compostela. He was never beatified, but when he died his relics were preserved, and stored in the cathedral of Saint James the Moorslayer.'

'I have always been faintly revolted,' Orm said, 'by the Christian obsession with bits of their holy dead.'

'Well, you should be glad of it. For, much later, when al-Mansur raided Santiago-'

'He who stole the cathedral bells.'

'Precisely. During that same raid he made off with the relics of al-Hafredi of Poitiers and brought them to Cordoba. So I came across the relics, as I followed hints of the story of al-Hafredi in other accounts – and so I eventually found the testament he left behind.' Sihtric stroked his bit of old skin.

'His testament. His story of how he came to Odo.'

'Yes. But there is more, Orm. In his testament Al-Hafredi goes on, beyond the events of the battle itself. He tells of another history. A history that would have come about if he had not come to comfort the defeated, suicidal Odo that dark October night. A history in which the Moors did not lose at Poitiers.'

This was the true history of the world, attested al-Hafredi of Poitiers, as it had been taught to him. It was a history in which no monkish wanderer had come to turn Odo's head.

Without the encouragement of the mysterious Alfred, Odo of Aquitaine surrendered to Abd al-Rahman, who used him as a hostage before casting him aside. When Charles of France faced the Moors without Odo, his numbers were not significantly weakened – but without Odo's whisperings about Moorish tactics he had a much poorer idea of the nature of the force he was facing. That inadequate knowledge led to crucial indecision. The Frankish force, rather than holding against the onslaught of the Moorish cavalry, broke and fled, as had all the Christian armies Abd al-Rahman had faced before.