The Crusades not only satisfied medieval Christians' genuine desire to spread and cultivate their religious beliefs, they helped funnel the restless energies of generations of well-trained, bellicose young men away from stable communities. No need for bored knights to hang around and make trouble in your kingdom--send them to the Holy Land! Give them a useful occupation. And for three Crusades' worth, this strategy worked well. Warfare took place in distant lands, with the opposing dead limited to Jews, pagans, and infidels.
The Fourth Crusade, however, irrevocably changed the medieval landscape and, for all useful intents, ended the Crusades. Yes, other half-hearted crusades followed in the centuries to come, but they never gained the same mythic, unanimous support found in the 12th century.
So what happened? Pope Innocent III initiated the Fourth in 1199 as a means of conquering Muslim Jerusalem via Egypt. To do that, the crusaders needed ships. It took two years to raise a significant army and negotiate naval transport with Venice, for interest had lagged after the failure of the Third Crusade. Eager for their promised 85,000 silver marks, the Venetians threw themselves behind this project, creating manned ships enough to transport 35,000 crusaders. But only 12,000 arrived in Venice, bearing only 51,000 silver marks.
Oops. What's a crusading army to do?
Attack Hungary! Yes, the Venetians said the debt would be paid if the crusading army attacked a rival port city, namely Zara in Dalmatia, allied with Hungary. The Hungarian king, Emeric, was Catholic and had volunteered to join the Crusade, but suddenly he found his territory under attack. Citizens of Zara hung crosses out their windows, to no avail. The city fell in 1202 with massive Hungarian Catholic casualties.
The head of the crusading army, Boniface, Marquess of Montferrat (west of Venice), left to meet with a relative, Alexius IV, son of the deposed Byzantine ruler Isaac II. Alexius proposed a massive bargain if the crusaders attacked Byzantium and deposed his uncle from the throne: "200,000 silver marks, 10,000 men to help the Crusaders, the maintenance of 500 knights in the Holy Land, the service of the Byzantine navy (20 galleys) to transport the Crusader Army to Egypt and the placement of the Greek Orthodox Church under the Roman Catholic Church." Too good to refuse!
Instead of heading to Egypt, the crusaders sailed to Byzantium and demolished the city over two years. Meanwhile, Alexius could not raise those 200,000 silver marks and had to melt down valuables throughout the city to raise funds, a sure sign of weak leadership. Fighting continued until the murder of priests and the raping of nuns became the sole objective of the marauding, disparate, directionless men. Tens of thousands were killed, many more were left homeless after innumerable fires, and looting stripped the grand city of its riches.
The outcome? None of the men of the Fourth Crusade ever made it to Egypt, let alone the Holy Land. Thousands were excommunicated by the pope who initiated the whole affair, and the Great Schism of 1054 between the Eastern Orthodox and Roman Catholic churches was made permanent. Eight hundred years later, Pope John Paul II twice apologized for the atrocities committed by the crusaders in the name of the Church. The Fourth was the last Crusade directly managed and sanctioned by a pope, and then only just. Future crusades were conducted outside of official religious dictates and lost, forever, any measure of genuine spiritual intention.