While the Battle of Yarmouk set the stage for hundreds of years of Byzantine antagonism with the Muslim nations of the Middle East, their next huge reversal came at the hands of their fellow Christians. Relations between the Latin and Orthodox churches over the centuries had been strained at best and openly hostile at worst. The initial split had been following the Council of Chalcedon, which declared that Christ had both a complete human nature and a complete divine nature. The Orthodox Churches maintained that Christ had a single nature, composed of both the human and the divine. While this may seem like an arcane point to us today, similar disagreements lay behind almost all of the schisms and heresies of the first five hundred years of Christianity’s existence – Aryanism, Nestorianism, Monophysitism and a dozen variants thereof. The overall consequence of this disagreement, however, was the division of the Christian world into two roughly equal halves – one known as “Latin” Christianity, the forerunner of the modern Roman Catholic religion, and the other as “Orthodox” Christianity, the forerunner of today’s Greek Orthodoxy, Russian Orthodoxy, and several other communions. This modern day descent is a lcue to a further difference between the two. While the Orthodox churches retained the structure of multiple Patriarchs of theoretically equal rank, the Latin Church consolidated itself behind a single leader – the Patriarch of Rome, who took the title of “Pope” (formerly an equivalent term to “Bishop” or “Patriarch”) and made it a title. Over the centuries following the fall of the Western Roman Empire, the Papacy gradually evolved into a political entity in its own right – and one the kingdoms of the West took great care to placate, as it held a great deal of power. Meanwhile in the East, it was the Emperor who took on the role of leader of the Byzantine Church, with the Patriarch of Constantinople serving beneath him. Given the subservience of Church to State in Constantinople, then, it is entirely understandable why Alexios Komnenos made the mistake he did in 1095.

The Battle of Manzikert, by Józef Brandt

Following the Battle of Manzikert, where the Seljuk Turks had defeated Byzantium and captured the Emperor Romanos IV, the Byzantines had been on the back foot. Much territory that had always been theirs was lost, and the Turks had even made it as far as the city of Constantinople itself. In the hope of recruiting Norman mercenaries, always a welcome addition to bolster his armies, Emperor Alexios sent a letter to Pope Urban II asking him to circulate his request for mercenaries. What he got was something else entirely. The Pope convened a council in his homeland of France, and there he preached of a new kind of war, a pilgrimage not in peace and humility, but of war and blood. What Alexios Komnenos’ letter produced was a war born not from political or economic concerns, nor a war of religious conversion, but something new – a campaign for territory born from religious principles, and the notion of a divine writ to the land. The Crusades had begun.

Ironically, given Byzantium’s role in sparking the Crusades, their involvement in the first three crusades was largely tangential. While they provided military support to the First Crusade, their aid was withdrawn at enough key moments to heighten tensions between Latins and Orthodox Christians – as was the crusaders’ treatment of the native non-Latin Christians of the Middle East. Relations between Byzantium and the new Crusader Kingdoms of Edessa, Antioch, Jerusalem and Tripoli were thus strained. When the fall of Edessa prompted the Second Crusade, its failure was at least partially blamed on deliberate sabotage by the Byzantines. The heightened tensions boiled over in Constantinople in 1182, when the regent Princess Maria, mother of the infant Emperor and a Latin, was overthrown by another member of the Komnenos family. Her pro-Latin policies led to a backlash that saw 80,000 people massacred by the mob. The Third Crusade even saw the Byzantine Emperor secretly ally with Saladin (Sultan of Egypt and leader of the military alliance against the Crusaders) to deliberately impede the Crusade. It is against the backdrop of this century of conflict, therefore, that the Fourth Crusade begins.

The Fourth Crusade is definitely the oddest of the Crusades, primarily because it never actually entered into any conflict with a non-Christian enemy. The Crusade was called in 1199, and the troops assembled in Venice in 1201. There, the Venetians had bankrupted themselves building a fleet to transport them, only to find that the actual Crusade was one-third the size they had anticipated. In fact, the assembled Crusaders were not even numerous enough to pay the agreed on fee for transport. As a result, the Venetians wound up using the Crusaders as private mercenaries for a couple of years. While this was happening though, a relative of one of the Crusade leaders met with a Byzantine prince in exile, named (as had been his ancestor who sent Pope Urban that fateful letter) Alexios Komnenos. He was the son of the previous Emperor, who had been deposed by his brother. Now the prince sought the aid of the Crusaders, to topple his uncle and put him on the throne.  (Standard practice for the Byzantines, this rejection of primogeniture (the succession of the elder son) was practically heretical in the eyes of the Westerners.) He offered large amounts of money, aid for the Crusades, and the submission of the Orthodox Church to the Pope’s authority. There was little chance of him delivering on this of course, but the Crusaders were won over, and sailed for Constantinople, bringing us (finally!) to the First Siege of Constantinople.

Constantinople. Note the chain across the bay marked in the northeast of the city.

Constantinople had been besieged before, by the Russ and the Turks, but both had broken off their sieges. This time would be a different story. The Crusaders first attacked a watchtower on the far shore of the river Bosporus from the city. This watchtower controlled the far end of the Great Chain, a key defensive mechanism for the city that could be raised to block passage up the river. They captured the tower and dropped the chain, allowing their galleys access to the lightly defended flank of the city. The Crusaders then launched a two-pronged assault via land and sea, and despite being outnumbered by the Byzantine soldiers they triumphed. The sitting Emperor, Alexios III, fled the city and the local people freed Prince Alexios’ father Isaac from prison and declared him Emperor. The Crusaders then forced him to declare Prince Alexios co-emperor. So ended the first siege.

Naturally, an Emperor placed on the throne by Latin Crusaders was not going to be popular with the Byzantine people. Riots frequently broke out between different factions, and the efforts by Prince Alexios (now Emperor Alexios IV) to raise the money to pay the Crusaders the funds promised did little to endear him to the people. Finally a courtier named Alexios Doukas took advantage of the situation to take over. He imprisoned both Isaac and Alexios IV, and declared himself Emperor Alexios V. The deaths of Isaac and Alexios soon followed. The hated Alexios IV was executed, while the more respected Isaac was said to have conveniently died of old age. The Crusaders were aghast at this train of events. Here were the rightful rulers of the kingdom imprisoned and killed by a man with no direct link to the throne, who nonetheless proclaimed himself Emperor and was accepted as such. Worst of all, Alexios V had no intention of honouring the promises of Alexios IV. This, of course, wouldn’t do at all, and so the Crusaders declared war on Byzantium. Less than a year after the first, the Second Siege of Constantinople began.

The Crusaders had a strong position surrounding the city, but Alexios V was made of sterner stuff than Alexios III, and the strong resistance of the Byzantine military surprised the Crusaders. Byzantine projectiles destroyed many of their siege engines, and the sudden severe bad weather that drove back one of their assaults had them doubting their faith in their cause. The clergy with the army acted swiftly, and spun the setbacks as “tests of their faith”, a common tactic to explain these reversals. (It’s fairly clear that the clergy were firmly in the pocket of the army’s leaders. A series of horrified letters from the Pope condemning the actions of the Crusaders throughout their campaigns had been suppressed by the leaders, and his condemnation of this siege was no exception.) Finally their luck turned, and they breached the walls, swiftly overrunning the city.

Entry of the Crusaders into Constantinople, by Eugène Delacroix

For three days, the Crusaders ran wild in the city, systematically looting churches and monasteries, murdering Orthodox clergy, and destroying priceless works of art to melt down for the precious metals they were constructed of. (It was only thorough the presence of the Venetians that some of the art was saved, as they appreciated it for itself enough to steal it intact.) The Library of Constantinople, one of the great storehouses of knowledge in the world, was destroyed. Thousands of people were killed, and 900,000 silver marks worth of treasure was looted. (Around $810 million in today’s money.) The jewel of the Byzantine Empire was shattered. The Byzantine Empire was (for the moment) defunct, and in its place stood the Latin Empire, the Crusader Kingdom of Byzantium. So ended the Fourth Crusade, the Crusade of Christian against Christian.

In the aftermath of the battle, Alexios V fled, and sought sanctuary with fellow exile Alexios III. Although the two initially got on well, the elder Alexios soon betrayed the younger, leading to his capture and execution by the Latins. Alexios III did not last much longer, however. The newly formed Latin Empire did outlast them, but the Crusaders found themselves facing not only the Byzantines’ old enemies in Bulgaria and Turkey, but also resistance from the unconquered Greek territories. It was the troops from these territories who finally reconquered Constantinople fifty years later and re-established the Byzantine Empire.

The long term effects of the episode were immense. Byzantium was weakened, fatally so. While it would manage to hang on for another two hundred years, it never truly recovered from the Latin mismanagement. Their distrust of the Greeks led to them demolishing the economic administration for the territories they controlled, crippling both trade and revenue. The schism between the Orthodox and Latin churches, meanwhile, was now firmly established in blood – so firmly that attempts by the Papacy to send assistance to Constantinople just before it finally fell were firmly rejected. Only in 2004, with a formal apology from Pope John Paul II, and an acceptance by Patriarch Bartholomew I, could the rift be said to be healed. But perhaps the greatest blow to the Empire was that the power of the capital stood exposed. The unconquerable walls of Constantine’s New Rome had been conquered. The Empire was teetering on the brink.