Fourth Crusade

The Fourth Crusade was an attempt of the western European Crusaders to conquer Jerusalem, which was under the control of the Muslims. However, instead of Jerusalem, the Crusaders sacked the capital of the Byzantine Empire, Constantinople.

Because of the Fourth Crusade, the Assassin Altaïr Ibn-La'Ahad was forced to cut his stay in Constantinople, where he hoped to spread the Assassin ideology, short and he returned to Masyaf.

Reference

 * Assassin's Creed: The Secret Crusade