Answer:
1. The Fourth Crusade, launched by Pope Innocent III in 1202, was originally intended to recapture Jerusalem from Muslim control. However, due to political and economic factors, the Crusaders were diverted to Constantinople, which was then the capital of the Byzantine Empire.
2. In 1204, the Crusaders succeeded in capturing Constantinople after a lengthy siege. They sacked the city, causing extensive damage to its architecture and looting many of its treasures. This event marked a significant turning point in the history of the Byzantine Empire, which never fully recovered from the devastation caused by the Fourth Crusade.