The Fourth Crusade, in route to Jerusalem, enters into an agreement with the Byzantine prince Alexios Angelos on January 21, 1203, to divert their main force to Constantinople to restore his deposed father, Isaac II Angelos, as emperor.
Both promising to add his support to their invasion of Jerusalem and pay their debts to the Venetians for their fleet. On June, 23 1203, the main Crusader army reached Constantinople, starting their siege on July 17, 1203.