History of the Crusades/The Fourth Crusade

Background
The Third Crusade was seen as a failure due to the fact that it failed to recapture Jerusalem, despite having restored many cities to Christian control. As a result, another crusade was called by Pope Innocent III to capture Jerusalem. However, the tactical goal of the crusade was to first capture Egypt before proceeding to Jerusalem. The crusader leaders overestimated the number of soldiers that would show up on the crusade, and as a result they did not have the money to pay for the Venetian fleet they had requested. So, the crusaders needed to repay their debt and in exchange they promised to aid the Venetians.

Events of the Crusade
The city of Zara had rebelled against Venice, so the Venetians asked the crusaders to subjugate Zara. The pope threatened excommunication to those who attacked Christians, but nonetheless the crusaders attacked and sacked the city. They were excommunicated, but the excommunication was later rescinded. Then, Alexios IV, a prince of the Eastern Roman Empire, offered to pay the crusaders' debt if they helped restore him to the throne. While many of the crusaders were disgusted by this and left, many continued to Constantinople.

During the siege of Constantinople, Alexios III fled and Alexios IV was crowned co-emperor. The crusaders no longer had any pretext to attack, but Alexios IV was unable to pay the money he had promised. As Alexios IV attempts to raise money alienated the populace, and soon he was overthrown by Alexios Doukas, crowned Alexios V. Alexios V refused to pay the money and the crusaders attacked the city, sacking it.

Constantinople was looted and many of its citizens were killed. A Latin state was then established with control over Constantinople while the rest of the empire dissolved. The Fourth Crusade caused massive damage to the Eastern Roman Empire from which it never recovered.