Let's review what we have learned so far. Jerusalem was conquered by Islamic armies in the 7th century. It was regained for Christendom by the First Crusade in 1099. In 1187, during the Second Crusade, Jerusalem fell to the Muslim Saladin. The Third Crusade (1189-92) was another recovery attempt, but ended in a stalemate.
The strategy of Fourth Crusade was to strike at Egypt, the base of Muslim power. It was conceived in 1199 at a jousting tournament in northern France where knights and barons swore solemn oaths to go as armed pilgrims to gain the land back from the Muslims. Rather than wear out their army by a long land march through hostile territory, the leaders decided to reach Egypt by sea.
A delegation of knights went to Venice, the leading seafaring city of Western Europe, to arrange for passage. Venice agreed that they would provide transport ships, crews and a year’s provisions for a total of 33,500 men and 4,500 horses, in return for a half-share of all conquests.
During the late spring of 1202, the crusaders began to gather at Venice. By the intended departure date their host totaled some 10,000 men, far short of the 33,500 planned for, and too few to provide the agreed upon charter fee. The Venetians had suspended their regular commerce to build and equip an immense fleet. Now they demanded that the crusaders hold up their end of the deal: 84,000 marks, or no crusade. The Fourth Crusade seemed on the point of collapse. Then the Venetians made the offer would suspend the unpaid balance of the transport charge in return for assistance in conquering the city of Zara, a Hungarian-owned port on the Adriatic coast. The crusaders were split on this matter, some feeling that it was an unholy act of war against fellow Christians, but others saw no choice if the crusade was to go forward. Although many knights deserted, in the end, most were persuaded to proceed. On November 10, the fleet reached Zara, which surrendered after a 14-day siege. Pope Innocent III excommunicated the Venetians and threatened to excommunicate the entire crusade. The crusaders set up winter quarters at Zara, as it was too late in the season to go on. There, the leaders met with Prince Alexius, who was the teen-aged son of the deposed Byzantine Emperor Isaac II escaped from captivity in Constantinople. The relations between Byzantines and Western Christians had deteriorated steadily through the century of the crusades, over which they were often at odds. From a Western viewpoint, an emperor who owed his throne to crusaders might be more cooperative. And so, the new plan was that the crusaders would stop at Constantinople on their way to Egypt, overthrow the usurper and put the young Alexius on the imperial throne. In return for the crusaders’ aid, he promised to pay off their debt to the Venetians and lead a Byzantine army in the proposed assault on Egypt.
In the spring of 1203, the crusade set out from Zara to the city of Constantinople (today’s Istanbul, Turkey). Constantinople withstood two epic sieges by the Muslim Arabs, from 673 to 678 and in 717, and other sieges by Avars, Bulgars and Russian Vikings so they were prepared for a seige both in terms of fortifications an in terms of the strong Byzantine army. However, the crusaders attacked both my land and sea, the Venetians against the harbor wall and the French against the north end of the land wall. The Venetians, once they were successful in their assault from floating siege towers, advanced to the shores set fire to the buildings, and driven by the wind, the fire then burned much of the city. The French attack on the land wall did not go as well. Word of the French peril reached the Venetians and so their soliders were redeploy in support of their allies. Constantnople, as a compromise, agreed to let young Alexius be crowned beside his father as Emperor. The crusaders agreed. It was, by this time, too late in the season to go on, but the crusaders looked forward to receiving supplies and Byzantine reinforcements. Come spring they could sail on to Egypt and retake the Holy Land.
Young Alexius, however, could not keep the grand promises he had made, as the imperial treasury was empty. The relationship between the Byzantines and the crusaders grew steadily worse. The throne was taken over by noble adviser, who kicked out the intruders and took steps to defend the city. He also took offensive measures, launched a fireship attack against the Venetian fleet.
The crusaders resolved to conquer the city and take the entire Byzantine Empire for themselves. The French were persuaded to join them in another amphibious attempt. After several hours and no success, the crusaders were forced back, and the fleet retired.
They regrouped and attacked again. This time Constantinople suffered a thorough and ruthless sack. While the French knights and men-at-arms went on a drunken rampage, the Venetians set to work like seasoned professional thieves, scooping up the best of the fallen city’s treasures.