The great medieval historian Steven Runciman wrote: “There was never a greater crime against humanity than the Fourth Crusade.” He was referring specificially to the Crusaders’ sacking of the city of Constantinople of 1204 and their overwhelming of the Byzantine Empire thereafter.

In his great work The History of the Crusades, he made clear his feelings on the crusading project as a whole that as great a victim of the Western European “barbarians” as the Muslims of the Middle East were the sophisticated, educated Greek Christian population of Byzantium. But how did the Crusading movement, which existed to save Christians from the Muslim advance in the Middle East, come to so devastate perhaps the greatest Christian city of the Middle Ages that Runciman could say there was never a greater crime in history?

In our time, many people view the Crusades very negatively, as an act of violent hypocrisy which has left a terrible historical legacy permanently contaminating relations between Western Christians on one side and Muslims, Eastern Christians and Jews on the other.

The Crusades began in 1096. At the time, Western Europe was in the middle of an intense religious revival, inspired by church reform and social changes. The Crusaders swept across Europe in the last years of the 11th century, almost maddened with fervour, slaughtering thousands of people and themselves dying in great number.

Jews in particular were the focus of Christian violence, and the Crusades are credited with the bloody expansion of antisemitism in medieval European society.

The First Crusade captured Jerusalem in 1099 and slaughtered the entire Muslim and Jewish population of the city. After this, however, a permanent Crusader culture developed in Palestine, which for many years survived in part because their Arab neighbours were themselves very disunited.

However by the end of the 12th century, the Arabs began to become resurgent and capture more and more territory from the settled Crusaders. The Third Crusade in the 1190s saw one of the most famous conflicts of the whole movement, between Richard the Lionheart of England and the Egyptian ruler, Saladin, who recaptured the city of Jerusalem.

The Third Crusade, riven with internal conflicts, failed to halt the decline of the Crusades and so exacerbated the poor relations between the major European monarchs that at its end, there was little appetite for any more crusades.

But Innocent III became pope in 1198. The preaching of a new Crusade was a major goal of his. Innocent realised two things: firstly, that Jerusalem was not a great enough prize to tempt the greed of the kings; and secondly, that the secret to shattering the Arab resurgence was to hit rich, powerful Egypt. These two realisations led to focusing on a new strategy for the Crusades: attacking Egypt.

What has this to to do with the sack of Constantinople? It mattered because now the great Italian trading cities of Genoa and Venice, bitter enemies in who was commercially the most important throughout the Mediterranean, became involved.

Genoa dominated European trade with Egypt and was opposed to any such attack. However, Venice saw an opportunity to take on their competitor and agreed to sail a Crusading army across the Mediterranean. Venice was not trying to break Egypt, though; it was trying to break Genoa.

Eventually, an army of holy warriors assembled in Venice in 1202 but one which was small in number and with hardly any money. The following year, they set sail to attack Cairo. The Pope sent a stern warning that no Christians should be attacked.

The Venetians agreed to help the Crusade in return for payment but quickly it became apparent that the army could not afford to pay what was owed. The Venetians, fearing a loss of face – and possible violence – if the Crusade they had agreed to help foundered, came up with a solution to clear their debt.

The Crusaders would attack Venice’s trading competitors in the Adriatic Sea instead, starting with the city of Zara, a prosperous port belonging to the Kingdom of Croatia. Despite the Pope’s ban on attacking Christians, Zara was captured with a large loss of life.

The warriors for Christ had become mercenaries for Venice.

Appalled, Innocent III excommunicated the Crusaders. Terrified of what would happen if the army found out they had not been saved but potentially damned, the leaders decided not to inform their followers of this.

And now Byzantium made a mistake.

Its emperor Alexius IV had long been involved in a struggle for the imperial throne with a rival, Isaac II. Now Alexius offered to pay the entire debt owed by the Crusaders to the Venetians, and more, if they helped him depose Isaac.

The Pope sent strong words that the Crusaders should proceed as planned to attack the Arabs, but instead, money talked. They accepted Alexius IV’s offer.

The city of Constantinople was only ever lightly defended. It had a long history of bloody intrigues, in which very large numbers of its emperors were murdered in coups, so it was often better to keep the army away from the city. Thus, when the Crusaders arrived in 1203, the city was very vulnerable.

The Byzantines failed to defend the city and the Crusaders marched straight into Constantinople, placing Alexius IV back on the throne in a compromise which made Isaac II’s son co-emperor. But there was one flaw in Alexius’s plan: he too did not have the money to pay.

He forced the population to melt down their holy icons to pay off his army of what were effectively mercenaries. The record of Byzantine emperors being murdered by rivals and the city’s mobs was not good, and now Alexius grew frightened. In fear for his life, Alexius offered the Crusaders more money to stay on.

Finally, in early 1204, as the Byzantines were finally getting fed up with the Crusaders, Alexius IV was strangled. Now with no hope of being paid, the Crusaders openly attacked the city for their own ends.

The Pope’s representatives demanded they stop and they were ignored. In April 1204, the Crusaders seized control of the city once and for all, beginning a savage three-day sacking, during which many ancient and medieval works of art were either stolen or destroyed.

The magnificent Library of Constantinople was destroyed and many beautiful churches were desecrated and burned. Large numbers of people were raped and murdered.

The sack of Constantinople by the Crusaders became one of the great scandals of medieval Europe. The Fourth Crusade had not saved Jerusalem at all. It had raped, murdered and stolen from the Christians of Byzantium.

The Byzantine Empire more or less collapsed, and those who had stayed on in Constantinople carved up its possessions between them. The so-called Latin Empire lasted another 57 years, but when Byzantium re-emerged, it was irrevocably damaged.

Other than the invasions of the Turks themselves, the events of 1204 have been seen as the most important factor in the eventual demise of the Byzantine Empire.

However, Pope Innocent, who had so valued the Crusading project and had excommunicated the Crusaders who had attacked Zara, now shamefully forgave the holy mercenaries. Returning Crusaders lavished him with the stolen riches of Constantinople and he in turn lavished them with praise.

Undoubtedly, the events of 1204 were a low stain on the history of both the Crusading movement and the Papacy itself. The impact on Byzantium was undoubtedly enormous and one could claim showed the growing moral corruption in the Church which would unravel more and more over the next 300 years.

But the Crusades were a symptom of life in this era. Indeed, was the devastation of Constantinople any worse than the slaughter of every Muslim and Jewish man, woman and child in Jerusalem a century before?

Or was Steven Runciman perhaps more concerned with the loss of artefacts and libraries than the loss of human life, which was, from the Jews of Germany onwards to the Greek of Byzantium and the Jews and Muslims of medieval Palestine, rendered so very cheap by the Crusades themselves?

You can read more about the Crusades in our upcoming book, A Quick History of the Crusades, which is published later this year. Sign up now to receive a free copy of the book before publication or to receive news about our other books.

We’d like to recommend some other books about the Crusades too. Why not start with Steven Runciman’s great, authoritative History of the Crusades? For something more unique, we love the novelist Amin Maalouf’s fascinating The Crusades Through Arab Eyes, which offers a thrillingly new perspective. Which books about the Crusades do you recommend or would like to read?