Peter Jackson, emeritus professor of medieval history at Keele University, came out with his magisterial The Mongols and the West, 1221-1410 in 2005, detailing the epic clash between the forces of 13th century Christendom and the waves of Mongol invasion threatening to engulf it. The standard account of that invasion has been the stuff of films and historical melodrama for 600 years: the brutish Mongols slaughtered whole populations of city and countryside with comprehensive gusto, sparing an assortment of accountants and clerks to run the administrative tasks to which they themselves were indifferent. The signature of the great Mongol warlord, Genghis Khan, was one of ruthless bloodshed.

In his earlier book, Jackson sought to add nuance to that standard account, using a wide array of sources to reinforce a more balanced picture of what might at first seem the least-reclaimable item in all of human history – the conquering Mongol horde. And his deeply impressive new book, The Mongols and the Islamic World: From Conquest to Conversion, continues that reclamation process, following the forces of Genghis Khan as they enter and overrun Central Asia in the early 13th century, quickly conquering virtually all Muslim territories east of Syria. Present-day Iraq, Iran, Turkey, Afghanistan – huge tracts of Western Asia fell under Mongol domination in the years that followed.

In these 400 densely-packed pages (followed by more than 100 additional pages of notes), Jackson concentrates primarily on two large questions: how did the Mongols manage to conquer so much territory so fast, and what was the experience of their conquest like for the rulers and inhabitants of the conquered lands, including the powerful Khwarazmian dynasty that covered Iran and chunks of Central Asia?

It’s a big story and Jackson tells it with great clear-headed energy and immense learning. And given the state of world affairs, it’s a particularly compelling story for 21st-century students of current events. “For the average westerner today,” Jackson writes, “the first assault on the Islamic world, by Genghis Khan’s Mongols in 1219-24, is just a part of a bigger process that seems to exercise a growing fascination: the rise of a hitherto obscure people, under a charismatic leader, to create the largest continuous land empire in the history of the planet.”

Jackson relates the latest scholarship dealing with these waves of invasion, pointing out a trifle optimistically that even “the Khans themselves have undergone a certain measure of rehabilitation”. He prefaces his book with a very helpful account of the earliest sources we have for any of this, including many works of Sunni Muslim scholars, a “largely untapped Persian source” called the Akhbar-i Mughulan, attributed to Qutb al-Din Shirazi, and the only Mongol source, the epic Mongghol’un niucha tobcha’an “the Secret History of the Mongols”. And he’s sensitive to the fact that historical accounts written by conquered peoples can sometimes be suspected of exaggerating the atrocities of their conquerors.

Those textual exaggerations lie at the heart of any attempt to rehabilitate the reputation of the Mongols, and a rigorous and far-seeing textual analysis forms a large part of Jackson’s book; it’s a good measure of the author’s virtuosity that he can make such analysis so unfailingly interesting. The relationship between conquerors and conquered is in these pages a far more complicated and thought-provokingly symbiotic one than readers usually get on this subject.

Jackson observes that the disunity of their enemies always contributed to the Mongols’ success, but he stresses throughout his book that another contribution was their ability to co-opt their victims in addition to merely destroying them. Chinese experts in siege warfare technology, for instance, greatly enhanced the Mongols’ ability to take even seemingly impregnable fortresses in Khwarazmian hinterlands. Likewise, Genghis Khan and his sons and top generals not only carefully studied their enemies before striking (a precaution that would cost the chaotic West dearly by facilitating a string of easy victories extending right up to the gates of Vienna) but tried to coerce as much help from those enemies as possible – something that certainly played a part in the conquest of the world of Islam, the Dar Al Islam.

“This was at no point an enterprise undertaken exclusively by the various steppe nomad or forest peoples he had conquered in Mongolia during the first decade of the century,” Jackson reminds readers. “His was not, in short, an army made up only of infidels, since from the very outset the Mongol sovereign drew on the assistance of Muslim confederates, who provided not just additional bodies of cavalry but infantry, an element lacking in the traditional steppe nomadic force.”

This is valuable historical reclamation work, and as an account of the ways the Mongol leaders mitigated the savagery of their own forces, it is surely correct. But as Jackson readily admits, such reclamation has its limits. The Mongols, as one Muslim historian put it, very likely killed more people than any other group in the history of mankind and they clearly relished the task. Cities in Khurasan, Khwarazm, Iraq, Mazandaran, Azerbaijan, Ghur, Bamiyan and Sijistan, among many other places, were inevitably subjected to extravagant violence, with Mongol forces reacting to any resistance whatsoever (and often no resistance at all) from target cities by slaughtering every man, woman and child in them.

Jackson mentions one such city, Nishapur, was “singled out for especially harsh treatment”, losing not only all of its human inhabitants to Mongol swords, spears and tortures but also all of its dogs and cats. Stories abounded of conquerors drinking the blood of freshly-decapitated victims, gradually dismembering people over the course of days, playing field games with severed heads, and so on.

And such trauma cast long shadows. Jackson relates the story of how the Dominican friar Riccoldo da Montecroce, sent to Iraq in the 1290s, was told the Mongols had conducted “such great slaughter, destruction and ruin” that nobody who had not personally seen it would believe it. Even allowing for scribal exaggeration, such stories were not told of other conquerors.

Khan’s grandson Hülegü famously sacked Baghdad in 1258 and ended the Abassid Caliphate that had ruled there for 500 years, a disaster so profound in the region’s cultural memory that, as Jackson points out, Osama bin Laden could reference Hülegü by name in 2002 and feel no further explanation was needed.

Jackson is consistently excellent at tracing such long influences, including, at the end of his story, the gradual conversion to Islam of several late-Mongol satraps (rulers) in the Ilkhanate.

The Mongol success story itself was doomed to fragment; they had no stable central authority, no structure of power other than relentless internecine factions and seemingly little interest in actually ruling the lands they conquered instead of only despoiling and beggaring them.

But as The Mongols and the Islamic World demonstrates with convincing assurance, the experience of Mongol aggression shaped the politics and society of medieval Islam far longer and more interestingly than either the conquerors or the conquered suspected at the time.

Steve Donoghue is managing editor of Open Letters Monthly.