The claim that the crusaders sacked Constantinople in 1204 is not the complete truth. The crusaders didn’t sack Constantinople. They engineered a regime change in the Byzantine Empire, at the behest of a powerful faction in Byzantine politics and some Venetian and French nobles. Here’s a brief account of the Fourth Crusade:
Pope Innocent III was elected on 8 January 1198. On 15 August 1198, he launched a new crusade (the fourth one). By the summer of 1200, a sizable crusading army had taken shape. Instead of taking a land route, which experience of the past crusades had shown was fraught with danger, the crusaders decided to take the route that Richard Lionheart and Philip II Augustus had taken during the Third Crusade. They planned to sail across the Mediterranean to Palestine.
The French Barons who were leading the Fourth Crusade entered into an agreement with Enrico Dandolo, the half-blind octogenarian Doge of Venice. Dandolo agreed to build ships for ferrying about forty-thousand crusaders, including the knights and their horses, across the Mediterranean for 85,000 silver marks. Pope Innocent III had exhorted the crusaders to conquer the Kingdom of Jerusalem, but without informing the Pope, the French Barons and Dandolo decided that the crusaders would conquer Egypt first.
The crusaders had to arrive at the Venetian port no later than June 1202, but most of them were late. By August 1202, only eleven thousand men had gathered, and to make matters worse, they didn’t have enough money to pay the shipping charges to the Venetians. The Venetians suggested that they would postpone the payment if the crusaders helped Venice conquer its enemy, the Croatian port town of Zadar. Zadar was under a Christian king, and several crusaders, who thought that they were going to fight for the Kingdom of Jerusalem, were appalled by the idea of attacking a Christian state. But most crusaders went ahead with the new plan. They conquered Zadar for the Venetians in November 1202.
From Zadar, the crusader ships should have gone to Egypt—that is what the general mass of crusaders had been expecting. But the crusaders found themselves in Constantinople. Who made the decision to navigate the ships towards Constantinople is one of the great mysteries of history. A powerful faction of Byzantine politics certainly had a role to play.
Alexius Angelus, son of the deposed and blinded Byzantine emperor Isaac II Angelus, had reached Europe before the Fourth Crusade departed from Venice. He offered to clear the Fourth Crusade’s entire debt to Venice, and equip the crusaders with enough manpower and funds to ensure the success of their campaign to conquer the Kingdom of Jerusalem, if they reinstated his father on the throne of the Byzantine Empire. He also offered to end the schism between Orthodox Christianity and Latin Christianity.
A deal must have been struck between some leaders of the Fourth Crusade and Alexius Angelus. This deal must have enjoyed Venetian support, since they controlled the shipping routes and without their cooperation the ships carrying the crusaders could not be diverted to Constantinople. The Venetians were in a position to gain a lot of commercial benefits if the regime change operation in Constantinople was successful. It is possible that the deal had German and French support since Alexius Angelus had meetings with German and French nobility before he arrived at Venice. Who was not part of the deal? Pope Innocent III and many of the crusaders.
On June 24, 1203, the Fourth Crusade had reached Constantinople. They demanded that Isaac II Angelus and his son Alexius Angelus should be restored to the throne. When Alexius III, who was then the Emperor, refused, the crusaders went on a rampage. In July 1203, Alexius III fled from the city. Isaac II Angelus and Alexius Angelus were proclaimed the joint rulers of the Byzantine Empire. The crusaders had accomplished their mission. But now a new problem arose—the two new Emperors were unable to honor the lavish promises that they had made to the crusaders and the Venetians. This led to a rapid deterioration in the popularity of their regime.
The two Emperors were arrested on 27 January 1204—apparently without the knowledge of the crusaders. Isaac II Angelus died soon after his arrest (perhaps due to poisoning). On 8 February 1204, Alexius III was killed by strangling. Despite the failure of Isaac II Angelus and Alexius Angelus to honor their commitments, the crusaders interpreted the deposition and murder of the two Emperors as a coup. They besieged Constantinople for more than a month. On 12 April 1204, they managed to enter the city. For three days, they rioted, vandalized, and looted, causing significant damage to life and property in Constantinople.
The two regimes that the Fourth Crusade toppled were Christian: Zadar and the Byzantine Empire. The Fourth Crusade never went to Palestine. The crusaders never marched towards Jerusalem. They stayed in Constantinople and founded a new kingdom to which they gave the name Romania. They first offered the imperial crown to Enrico Dandolo who refused it. Baldwin I was crowned as the first emperor of Romania on 9 May 1204. The surviving members of the Byzantine Empire’s aristocracy founded their own empire in Nicaea, where they awaited for an opportunity to win back the empire that they had lost.