It was not unusual for Henry to issue threats: they were fundamental to his arsenal of kingship, as vital as his carefully calculated thunderous outbursts, his diplomacy, the legendary speed at which he drove his armies and his unsurpassable siege warfare in inspiring awe among his adversaries. Henry did not discriminate between the recipients of his threats, from the pope to the lowly electors of Winchester, whom he once ordered to “hold a free election” but forbade “to elect anyone but Richard my clerk”.

But this was of a different order altogether. Since 1097, European crusaders had been fighting the forces of Islam in the Middle East and tenaciously hanging on to their conquests: the kingdom of Jerusalem, the principality of Antioch, the counties of Edessa and Tripoli. Muslims were seen as Christendom’s enemies.

Moreover, Henry was not simply King of England: he was also the Duke of Normandy and Aquitaine, Count of Maine, Anjou and Touraine, master of vast swathes of France. One of the world’s most powerful men, he held sway from the Scottish borders to the Middle East, where his uncles ruled the Latin kingdom of Jerusalem. If Henry was serious, the ramifications across 12th-­century Europe would be seismic.

Could this, then, have been more than Henry’s characteristic bombast? Is it possible that he meant what he said?

Henry was familiar with Islam. He would have studied the works of Petrus Alfonsi, physician to his grandfather Henry I, who wrote the earliest credible account of Muhammad, as well as Peter the Venerable, who ordered the first translation of the Qur’an into Latin. Although he saw Islam as a heresy, Peter thought it the greatest of all heresies – the one that most deserved to be answered.

Alongside Islam, Henry also developed an admiration for Arabic learning from an early age. He had received an outstanding education from scholars versed in the ‘new’ knowledge that was exploding out of Sicily, Spain and the Middle East. Western Europe had never experienced such an intellectually exciting period as the 12th century – later dubbed the 12th­century renaissance – fed by a rediscovery of the classical thinkers of Greece and Rome (particularly Christian Rome after Constantine’s conversion), and by contact with the Arab world and its rich intellectual tradition in astronomy, medicine, music, architecture and mathematics.

Henry’s parents – heeding the lesson from the monk William of Malmesbury that “a king without letters is [just] an ass with a crown” – had hired the best tutors in Europe. Among them was the renowned Arabist, linguist and scientist Adelard of Bath, who had a profound impact on Henry’s education. Adelard had travelled for seven years in Italy, Sicily, Antioch and the southern coast of what would become Turkey, dedicating himself to the ‘studies of the Arabs’. He was famed for his translations into Latin of Arabic treatises on astro nomy, and introducing Arabic innovations in mathematics into England and France. Adelard dedicated De opera astrolapsus – his work on the Arabic innovation of the astrolabe – to Henry.

Henry’s interest continued into adulthood. He welcomed travelling scholars, not least Arab ones, to his courts. He knew enough about Arabic learning to request specific texts from diplomats travelling to Sicily and the kingdom of Jerusalem. And Henry admired the Islamic arts so much that when he built a palace for his mistress Rosamund Clifford, at Woodstock, he mimicked the palaces of the Norman kingdom in Sicily, with fountains and courtyards. The palace was later destroyed but its style, abounding in Arabic motifs, was unique in northern Europe.

Why did Henry II threaten to convert to Islam?

So much for the king’s high regard for Islam and Arabic culture. But what was it that provoked Henry to make the threat in the first place? The answer is to be found in Henry’s letter, where he tells Pope Alexander he “would sooner accept the errors of Nur al-Din [the Sultan of Aleppo] and become an infidel, than suffer Thomas [Becket] to hold sway in Canterbury Cathedral any longer”.

Now things become a little clearer: it is 1168, and Henry’s row with his erstwhile friend Thomas Becket is in its fifth weary year. Henry had raised Thomas high, appointing him to the position of chancellor soon after his accession. He was “considered second only to the king”. Henry had such faith in Thomas to do his bidding that after Theobald, archbishop of Canterbury, died in 1161, he strong-armed a reluctant Becket into taking up the dual position of chancellor -archbishop, despite warnings from Henry’s mother, the Empress Matilda, and from Thomas himself. Thomas thought it was ludicrous, protesting that Henry and he knew “for certain that if I am ever promoted to that dignity, I will have to forfeit either the king’s favour or… my service to God Almighty”.

Henry ignored all objections, paying no heed to his mother and even threatening the monks of Canterbury (who did not want Thomas as their archbishop) with his anger if they failed to elect his candidate. Henry’s primary concern was to ensure the succession by crowning his eldest surviving son in his lifetime. It was his bid to avoid another blood-spattered race to the throne when he died – as had happened at the death of every monarch except Stephen since the Norman conquest. The right to crown the kings of England was the prerogative of the archbishops of Canterbury, and Henry expected Thomas to accede to his desire.

Instead, Henry discovered that he had installed a zealot, a soldier now for the eternal Christ instead of his temporal king. Henry was livid when Thomas resigned the chancellorship; king and archbishop soon became locked in a battle for supremacy between church and state. The balance of compromise – whereby the kings gave their archbishops dignity and in turn the archbishops sought to please their kings – was torn to shreds.

Thuggish barons

The major source of friction was over which court – the king’s or the church’s – should govern clerics accused of committing crimes. Henry was concerned that separate ecclesiastical courts operated in tandem to his own, but he was also vexed that the punishments they meted out were negligible. Neither king nor archbishop would budge.

When Henry, seeking to rid himself of Thomas, charged him with contempt of royal authority and embezzlement in 1164, Thomas foresaw his imprisonment, and even death. He was reminded by some of Henry’s more thuggish barons that the king’s own father, Geoffrey of Anjou, had castrated some of his clergy for their disobedience, forcing them to “carry their members” before him in a basin. Petrified, Thomas fled to the court of Louis VII of France, where he was gleefully offered sanctuary. Louis, the first husband of Henry’s wife, Eleanor of Aquitaine, was ever pleased to create trouble for his most powerful lord – Henry owed allegiance to the French king for his lands on the continent.

Louis, as pious as Henry was worldly, had offered sanctuary not only to Thomas; he had also given refuge to Pope Alexander III when he had left Rome in 1159 following a split election that resulted in the first in a series of antipopes to occupy the papal throne.

Pope Alexander also owed much to Henry, who had supported him alongside Louis. Alexander needed Henry’s backing, and the row over Becket put him in an impossible position. Although he may have sympathised with a bitter Thomas, he was compelled to tread a careful path. For the next few years he would procrastinate, even while allowing Thomas to vent about Henry’s destruction of the church through his “evil customs”. (As it turned out, Becket’s fears were justified. He was murdered two years after Henry composed his letter to the pope.)

Henry’s conversion threat was a bludgeon, waved before the pope the more forcefully to persuade him to remove Thomas from his post. He had successfully threatened Alexander before. Eight years earlier, he had sought a papal dispensation to allow his five-year-old son to marry Louis’s infant daughter Margaret, enabling him to grasp the Vexin, a key French county that was baby Margaret’s dowry. He had bullied Alexander’s ambassadors into thinking he would back the pope’s rival, the antipope Victor IV, if he did not get his way, and Alexander had capitulated. It had worked before, so Henry likely believed the pope would yield again in the face of his outlandish threat. As far as we are aware, however, Alexander did not respond to it directly, but continued to push for negotiations between Henry and Becket.

As to whether Henry would ever have carried out the threat, it is unlikely. A practitioner of realpolitik, he would have been all too aware of the dangers. His hold on power notwithstanding, Henry would not have been able to deny that his divine right to the crown of England sprang from Christianity. Christian society was structured in a very different way to Islam; it was primarily an agrarian and feudal society. Islamic society allowed for a reasonably high level of social mobility and was far less rigid than the Christian west’s feudalism. Henry’s empire was based on an intricate system of oaths and obligations.

Henry’s conversion would presumably have required the mass conversion of all the different peoples in the lands under his rule, from Northumberland to Aquitaine. The administrative implications alone would have been immense. What would have become of the thousands of bishops and priests? Would Arabic have replaced Latin as the lingua franca? Would there have been a new curriculum in the universities? Would Henry have developed Arabic rather than English law? With which caliphates would he have forged his new alliances? What would have been the effect on the crusades?

Consider the hundreds of years of chaos sparked by Henry’s descendant, the eighth Henry, with his separation of England from the Roman Catholic church – chaos that resulted in civil war and the eventual execution of a king. We can only imagine the bedlam that would have ensued had Henry II converted to Islam. Conversions in both directions did happen, particularly in the marchlands where Christianity and Islam met – but the converts were not kings or queens. Such an undertaking would have been beyond the diplomatic and administrative talents of even a king as exceptional as Henry II.

If there is anything to learn about Henry from the episode, it is perhaps that he cared far more for the temporal than the divine. Although a superstitious man, Henry was not a religious one. The chroniclers railed against his lack of piety, claiming that he never sat still in church. Henry found himself so bored at mass that he doodled and met petitioners. His threat to convert to Islam is indicative of how little religion meant to him and, as a result, how much he resented papal authority when it stood in his way. For all the reasons that Islam may have appealed to Henry, one of the most attractive would surely have been that, unlike Christianity, it had no centralised authority, no supranational power. How gratifying the thought: no Muslim pope to bar him from sacking his own archbishop.