This book opens disarmingly with a novice historian stumbling through a lecture, “wondering why he had ever begun”. He is saved by his students, who storm the hall and, with cries of “Deus lo volt”, demand to be taken to Jerusalem, AKA the pub next door. Thirty-six years later, Christopher Tyerman, now professor of the history of the Crusades at the University of Oxford, returns to the subject of his lecture: how to plan a crusade.

The first challenge for Pope Urban II when he made the call to arms at the Council of Clermont in 1095 was to convince western Christians that the “liberation” of Jerusalem and the defence of their co-religionists 2,500 miles away was a good idea. A sizable carrot was presented in the form of remission of all the penalties of confessed sins. In a divinely ordered world, where everyone believed in God and was terrified of purgatory, war had hitherto been seen as a sinful deed that required penance. Now, for those “took the Cross”, or who vowed to become crusaders, it was sanctified slaughter, a penitential act in itself. Crusaders were told that if they died in the service of the Cross they would vault over purgatory “as if in one leap they pass into heaven”. (This metaphor was popular in Flanders, where it was customary to pole vault over canals.)

According to the church authorities, Jerusalem was part of Christendom because it had been bought by the blood of Christ. It was both a relic of the past – the site of Christ’s life, death and resurrection – and the stage for the second coming and last judgment. The case for its urgent recovery was bolstered by millenarian rhetoric and inflammatory images of Muslim sacrilege – including, it was alleged, huge cartoons of Saladin’s horse befouling the Holy Sepulchre. Recruits were promised “paradise and honour / merit, reputation and the love of one’s beloved”. Those who seemed reluctant to sign up were likened to chickens confined to their hutches or freshwater fish fleeing the sea. During the campaign for the third crusade, suspected shirkers were sent wool and distaffs, a shaming device that anticipated the white feathers of 1914. Misogynistic pressure was put on wives, who (until Innocent III changed the rules) could refuse their husbands permission to enlist.

Preaching tours were well organised and well publicised, though inevitably some speakers fared better than others. John of Xanten talked with his eyes shut, Archbishop Baldwin of Canterbury was mobbed after an invigorating performance at Hay‑on‑Wye, and one Cistercian abbot discovered that he could only rouse his slumbering listeners by namechecking King Arthur. The response to the first call was terrific: perhaps as many as 100,000 people signed up within 18 months of the Council of Clermont. By the 13th century, there was hardly a region in western Europe that had not seen some sort of crusade promotion.

Although the chroniclers tended to idealise poverty and the poor (loosely defined as plebs, pauperes, peregrini and mediocres), crusading was a rich man’s game. The main targets of recruitment were those with liquid capital and large retinues. Royal endorsement helped: every king of France between 1137 and 1364, of England between 1154 and 1327, and of Germany between 1137 and 1250 took the Cross, which is not to say that they all ventured to Palestine. Vow redemption, a sort of get-out-of-crusade payment scheme, was a useful source of revenue. It was also open to abuse. By the early 14th century, redemptions had morphed into outright indulgences of the kind that would get Martin Luther so worked up in 1517.

Income taxes came with the Crusades, including the notorious Saladin tithe, which became a model for future parliamentary taxation in England. Crusaders were exempt. They also benefited from interest-free loans and other immunities and privileges: “Mammon in the service of God”, notes Tyerman.

Still, there was a hole in the heart of crusade strategy. Jerusalem and the Holy Land were easier to win than to keep. Thirteenth-century lawyers argued over the rights and wrongs of forced conversion (forbidden by canon law) and the seizure of territories that had never belonged to Christendom, but these were theoretical debates. The resources required for the permanent conquest of Egypt or anywhere in the mainland of the Levant simply weren’t there. “I will tell you what this is like,” said Erard de Valéry at the Second General Council of Lyons in 1274. “It is like the little dog barking at the great big one, who takes no heed of him.”

Then there were the other crusades – in the Baltic, Spain, Germany, France, Italy – where the concerns of Christendom might seem less important than those of regional politics. In 1213, the crusade hero Peter II of Aragon died fighting for control of Toulouse against Simon de Montfort, the champion of the Albigensian crusade. When men of the Cross fought other men of the Cross in wars of the Cross thousands of miles from the Holy Land, cynicism was a likely offshoot.

Facebook Twitter Pinterest Manuscript illustration depicting the taking of Damietta in the Nile Delta during the fifth crusade. Photograph: Corbis

Tyerman argues that the ultimate failure and (to most modern eyes) indefensibility of the Crusades should not detract from the feat of their realisation. They were “functionally effective”, he writes, “grounded in practical programmes of promotion, recruitment, planning and finance, less Don Quixote, more Dwight Eisenhower”. His crusade hero is not Bohemond or Richard I, but the humble quartermaster. He presents evidence for meticulous provisioning, contracts of service, parchment travel tickets, military ordinances, itineraries, trade licences, banking systems and so on. Crusaders were quick learners. They modified their ships for the long‑distance transportation of their warhorses and they recycled their timber to build on-site hospitals. They produced prefabricated castles (Richard I’s at Messina was called “Mategriffon”, or “kill the locals”) and designed floating fortresses: Oliver of Paderborn’s amphibious siege tower proved invaluable during the siege of Damietta in the Nile Delta in 1218. Progress was made in everything from taxation and trade to map-making and medical provision. And this is really Tyerman’s point: an age of faith is not an age of ignorance, still less of incompetence. Religion need not be antithetical to reason.

How to Plan a Crusade is serious and scholarly, the synthesis of decades of work on difficult, fragmented sources. Administrative records weren’t routinely kept until around 1300, which makes Tyerman’s task harder and more impressive. As its title and the hint of Lucky Jim in the preface suggest, this is also a lively book, laced with wry asides and enough surprising details to pique the general reader. I did not know that an autopsy performed on a slaughtered bear may have saved the life of Baldwin I of Jerusalem, or that laundresses doubled as de‑lousers on the third crusade, or indeed that stable-boys sailing to the Holy Land would have had to throw 25 kilos of dung overboard, per horse, per day.

We are constantly being told that the Islamic State in Iraq and the Levant is medieval in its outlook. It is invariably meant pejoratively, a stubby finger pointing at a group known for bigotry, obscurantism and bloodlust. This magisterial study takes us to the administrative and intellectual hinterlands of the 11th, 12th and 13th centuries, offering a finer, more expansive view. It reveals faithful minds more than capable of sophisticated planning, manipulative propaganda and the application of reason to religious warfare – areas in which Isis has shown a proficiency that is, indeed, positively medieval.

• Jessie Childs’s God’s Traitors: Terror and Faith in Elizabethan England is published by Bodley Head. To order How to Plan a Crusade for £20 (RRP £25) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.