In 520 BC, an inveterately fractious people named the Elamites rose in revolt against their imperial overlords, the Persians. The newly-crowned Great King, a usurper by the name of Darius, was in no mood to be patient. The Elamites, so he informed his men, were rebels against the Persian god of truth and light, Ahura Mazda. Anyone who fought against them might expect ‘divine blessings – both in life and after death’. Darius’s men, buoyed by these assurances, duly moved in for the kill. The rebellion was briskly crushed. The Elamites were left tamed for good. Such was the impact of the world’s first holy war.

It is no exaggeration to describe this otherwise obscure campaign as one of the most portentous in history. Darius’s vision was of an empire that fused cosmic, moral and political order – and it was one destined to enjoy a long afterlife. When the Great King anathematised the Elamites for their failure to worship his own god and condemned them as ‘evil’, he was articulating, for the first time, some fateful notions: that foreign foes might be crushed as infidels; that soldiers might be promised paradise; that warfare in the name of a deity might be cast as a moral duty. As an ideology, it helped Darius to seize and maintain the largest empire that the world had yet seen and his elaboration of it marks him as one of the supreme geniuses in the history of warfare. His insight – that violence can be rendered all the more lethal if sacralised – was to prove at least as potent on the battlefield as any tactical or technological advance.

Certainly, for almost two centuries, it would help to sustain his heirs in the rule of their stupefying dominions. It was not one, though, that the ultimate conqueror of the Persian Empire inherited. Alexander, the king of Macedon whose genius for warfare would ultimately lead him as far as India, relied on the blaze of his own charisma to inspire his followers and never thought to appeal to the sanction of the heavens. Instinctively, though, he appreciated the potency of the claim made by the Persian monarchy to serve as the agent of God – which was why, once he had humbled it amid the dust of battle, he also made sure to incinerate its prestige. In 330 BC, Alexander captured Persepolis, the sacral capital of Darius’s dynasty, and put it to the torch. The victory was not just over the defeated Persian king, but over his claim to a privileged relationship with the gods. The Macedonians, though, had nothing to put in its place. An empire that Alexander had been able to hold together by sheer force of personality immediately began to disintegrate, once he was dead. An assortment of his generals, grasping after such territories as they could seize, tore it to pieces. A philosopher, surveying the spectacle of self-made kings scrapping over the corpse of the Persian Empire, nakedly self-aggrandising and avid for loot, was moved to a sombre reflection. ‘Fortune, not reason, is what guides mankind.’ So mused Theophrastus, the heir of Aristotle, erstwhile tutor of Alexander. The mirror held up to the heavens by Macedonian warlords was one that showed in its glass only chaos and greed.

And when, in due course, the kingdoms founded by Alexander’s generals began to fall into the shadow of a new imperial power, there was little doubt that Tyche, the goddess of fortune, was to blame. ‘Since Fate has given to the Romans the empire and mastery of the world,’ declared the ambassadors of one Macedonian king, ‘they should show mercy and generosity in the employment of their good luck.’ This, however, was not at all how the Romans saw things. They took for granted that they were the most god-fearing of peoples. Foreign analysts, who tended to regard the Romans’ piety as ‘superstition’ and interpreted it as a subterfuge played on the masses by a cynical ruling class, misread its essence. ‘No just war can be waged’, declared the great orator Cicero, ‘unless it be to punish or repel an enemy.’ To those who had experienced the full terrifying force of the legions, the notion that the Romans might have conquered the world in self-defence was liable to seem a risible one – but not to the Romans themselves.

This was why, when their traditional republican system of government imploded in the first century BC and the very future of their city seemed at stake, amid the carnage of repeated civil wars, most looked to the heavens to explain what had gone wrong. Once, the gods had graced Rome with their favours and their protection. Incense had perfumed the flames of sacrifice and veiled the sun with smoke; the blood of white oxen had spilled from axe-blows onto the earth; festivals of primordial antiquity had given order to the city’s year. But then, over time, as the processions had come to be abandoned, so the rituals had been forgotten and the stones of the shrines grew mute. The poet Horace, who had fought in the bloodiest battle of the civil wars, was only one of many to shudder at the sight of temples sharing in the general dilapidation of the city. ‘The sanctuaries with their dark images stand ruined, befouled with smoke.’ Haunted by his memories of citizen slaughtering citizen and impoverished by the loss of his lands, Horace had drawn the obvious conclusion. ‘The gods, because neglected, have brought a whole multitude of evils on sorrowing Italy.’

Which was why, when Augustus Caesar emerged from the chaos of the times as the city’s ultimate strongman and established, amid the rubble of the Republic, an autocracy, he made sure to veil his supremacy behind an ostentatious show of piety. Crumbling and roofless temples were an affront both to the gods and to the dignity of the Roman people: pustules upon the face of the city. Augustus, with the wealth of the world at his disposal, could well afford the necessary medicine. What had been decayed was to become pristine; what had been black was to become white; what had been mud-brick was to become marble. By the end of his life, Augustus had restored no fewer than 82 temples. Meanwhile, far distant from Rome, legions stationed among forests or deserts, on barbarous frontiers, performed a similar feat of ordering upon the fabric of the world. The pax Romana was not a passive thing, but a martial, aggressive demonstration that Rome, once again, enjoyed the blessing of the gods. It was Horace’s fellow poet, Virgil, who gave the doctrine its canonical form. ‘These, Roman, are your arts: to impose the ways of peace, to show mercy to the conquered and to subdue the proud.’ The vision – of a world broken for its own good to the god-given fiat of Rome – was one that Darius might well have recognised.

Three hundred years on from the time of Augustus and not even a near-century of renewed civil war had shaken the conviction of the Roman people that eternal glory was their god-given due. Edward Gibbon, in The Decline and Fall of the Roman Empire, would see in the conversion of Constantine to Christianity a fateful moment of weakening, when the martial inheritance of Rome was sacrificed upon the altar of pacifism, thereby dooming the city to collapse. In fact, the opposite was true. When Christ first spoke to Constantine, He offered him victory, plain and simple – and was true to his word. Constantine himself, later in his life, would recall the experience of his conversion as a single moment of transcendent and soul-wrenching rapture; and yet, in truth, his spiritual journey had been altogether more protracted, perhaps, than such a memory allowed. Long before his vision of Christ, the would-be emperor had been casting around for a divinity sufficiently powerful to sustain the awesome scope of his ambitions. Such a god, his own quite stunning lack of modesty being what it was, had needed to be one of such might, of such potency, of such magnificence, as to reign alone. At various stages in his career, Constantine had imagined that this supreme god might be Apollo, or perhaps the sun. In the end, however, it was Christ who passed the audition. Constantine had called on the one God ‘with earnest prayer and supplications that He would reveal to him who He was’ – and a cross of light had duly appeared in the skies. From that moment on, the emperor never fought a single battle but he first retired to his tent and prayed – ‘and always, after a short while, he was honoured with a manifestation of the divine presence’. That this was more than a mere delusion was evident in the startling scale of his achievements. Who were his heirs to forget it? To rule in Constantinople, the great city founded by and named after Constantine, was to appreciate just what was owed by the Roman people to the favour of Christ. It was to know them protected by a guardian of incomparable power. It was to rejoice in the certainty that their empire, even amid all the convulsions and upheavals of the age, rested upon foundations of adamantine solidity: those same foundations that were the proper knowledge of God.

Christianity, far from gelding the Roman Empire, gave it the self-assurance to survive far longer than it might otherwise have done. Yet in the ideological arms race that was the clash of would-be universal empires in late antiquity, it was only a way-step. For all its success in buttressing the conceit of Caesars, it did indeed contain within its core doctrines a profound strain of pacifism. Soldiers, in the grip of their faith, might be induced to give away their military cloaks to beggars and retire from the army altogether; saints, abandoning the haunts of men, might retire to desert cells, there to serve as milites Christi, ‘soldiers of Christ’, and fight with demons. Time, though, would see a new faith arise, whose warriors would wield the sword for real and militarise much that, in Christian doctrine, had existed as metaphor. Muhammad, in his summoning of the Arabs to holy war, was recognisably bred of the spirit of the age. ‘In truth,’ it was revealed in the Quran, ‘the punishment of those who make war against God and His Messenger and roam the earth corrupting it is that they be killed, or crucified, or have their hands and feet amputated, alternately, or be exiled from the land.’ Such a message was all the more resonant, no doubt, for being delivered direct from the Almighty Himself; and yet, the sentiments informing it would have been instantly recognisable to anyone familiar with the workings of Roman power. As Constantine’s own victory in battle had illustrated, examples were hardly lacking of spectacular violence committed in the cause of God. Umar, who ruled as the second ‘Successor’, or ‘Caliph’, to Muhammad, pointedly turned the pretensions of the Christian empire back upon itself. To those he conquered, as well as to those he commanded, he seemed a warlord of a thoroughly recognisable kind. What added incomparably to his prestige, however, and did indeed suggest something novel, was that his earth-shaking qualities as a generalissimo were combined with a most distinctive cast of virtue. Rather than ape the manners of a Caesar, as earlier Arab leaders had sought to do, he drew on the example of a quite different kind of Christian. Umar’s threadbare robes, his diet of bread, salt and water, and his rejection of worldly riches would have reminded anyone from the desert reaches beyond Palestine of a very particular kind of person. Monks out in the desert had long been casting themselves as warriors of God. The achievement of Umar was to take such language to a literal, and previously unimagined, extreme. His brand of piety, like his brand of militarism, was bred of its time. A century on from the founding of the Caliphate, Christian ascetics continued to provide inspiration for Muslims scornful of idle pleasures and with an appetite for hardship. That this was so might be perfectly evident to monks and Muslims themselves. One traveller, happening to meet with a Christian hermit and marking how his eyes were puffy from weeping, asked him the reason for his tears. ‘Because the hour of my death is fast approaching,’ the monk replied, ‘and I still have far to go.’ Sometime later, when the traveller passed the monk’s cell again and saw that it was empty, he asked where the monk had gone. ‘He had become a Muslim,’ came the answer, ‘and gone raiding and been killed in the lands of the Romans.’

Yet there was, in this aping by Muslim warriors of Christian ascetics, a potential source of embarrassment. How could Islam, pledged as it was to the capture of Constantinople, possibly hope to scour the world of unbelief, when there still lurked in the souls of its own shock troops a lingering taint bred of their foes? Fortunately for Muslim scholars, however, the solution to this problem lay ready to hand. ‘The words of our Prophet have reached us – a correct and truthful statement.’ So declared Ibn al-Mubarak, a Turk whose relish for fighting the Romans had led him to travel to the Caliphate’s frontier with the Byzantine Empire, all the way from far distant Khorasan, and who would come to rank as perhaps the most formidable of all the warrior-scholars. The ‘imam of the Muslims’, as he was admiringly hailed, he possessed not only a rare aptitude for defeating Romans in single combat, but a familiarity with the sayings of the Prophet so detailed and passionate that he was known to discourse on them even in the heat of battle. Who better, then, to reassure his fellow Muslim warriors that the mortifications required of them on the frontier had in fact been authentically Islamic all along and owed nothing to the example of infidels? Why, Muhammad himself, so it suddenly appeared from a flurry of sayings attributed to him and brandished to triumphant effect by Ibn al-Mubarak, had given Muslims explicit instructions not to copy monks. ‘Every community has its monasticism – and the monasticism of my community is jihad.’

Quite what was being suggested by this word remained, however, in Ibn al-Mubarak’s own lifetime, something very much up for grabs. Its literal meaning was ‘struggle‘ and, in the Quran, reference to the jihad required of believers was as likely to imply a good argument with un-believers, or the giving of alms, or perhaps the freeing of a slave, as it was any commitment to pious violence. With warrior-scholars such as Ibn al-Mubarak desperate to claim the Prophet as their exemplar, however, the word came increasingly to take on a much narrower connotation: warfare in the cause of God. Riding to the frontiers of the embattled House of Islam and slaughtering stiff-necked Christians was cast not merely as an option for dutiful Muslims, but as a positive obligation. To one battle-shy friend who had boasted of his pilgrimages to Mecca and Medina, Ibn al-Mubarak gave a blistering retort. ‘Were you to see us,’ he lectured, ‘you would realise your worship is mere play. For you, the fragrance of spices, but, for us, the fragrance of dust and dirt and blood flowing down our necks – which is altogether more pleasant.’

A hundred years and more after the death of the Prophet, evidence for this robust approach to the essentials of Islamic worship was coming to be marshalled in impressive quantities. Ibn al-Mubarak himself compiled an entire book devoted to the single topic: jihad. Other scholars, turning to the Quran and finding themselves puzzled by the alternation of passages that urged perpetual warfare with others that seemed to urge the precise opposite, sought to organise the verses in what they trusted was a chronological manner – with one in particular (Quran 9.5), targeted originally at treaty-breaking unbelievers, being placed right at the end of the Prophet’s life. ‘Kill them, seize them, besiege them, wait for them at every look-out post’ – maxims with an obvious value for those with a taste for fighting Romans. It was therefore crucial for such scholars to establish that the verse was indeed revealed to the Prophet late in his career: for only so could it plausibly be demonstrated to have superseded other, less bellicose passages. As a result, it was not only collections of the Prophet’s sayings that were starting to be shaped by the martial enthusiasms of Muslim scholars, but details of his very biography. For scholars such as Ibn al-Mubarak, the stakes could hardly have been higher. Fail to demonstrate that they were following Muhammad’s example and not only their increasingly complex doctrine of jihad, but all their sufferings on campaign against the Romans, would effectively rank as worthless. Render the Prophet satisfyingly in their own image, however, and the prize would be a truly fabulous one. Not only would the past of Islam be theirs – its future would as well.

And so it has proved. The doctrine of jihad forged in the 8th century by itinerant Muslim scholars continues to exert its influence on the world 1300 years on. Yet the origins of jihad itself and crusades and even wars fought in defence of democracy and human rights all, in the end, and however indirectly, reach much farther back, to the very dawning of the dream of global empire. Ideas put to the test by Persian kings and Roman Caesars have proven far more enduring than either their armies, or their empires. The world today still remains in the shadow of the Elamite War.

This essay originally appeared under the title The Sacralisation of Violence in War: Perspectives from the Engelsberg Seminar, Axess, in collaboration with Axel and Margaret Ax:son Johnson Foundation, 2015.