(On March 11 JiaZi), April 12, 1204, the Crusaders sacked Constantinople.

medieval Crusades, the war of aggression of the big feudal lords of the Holy See, the Western European knights and eastern Mediterranean countries in the Near East countries. From 1096-1270, the war lasted for nearly two centuries old.
Why is it called the "crusader"? Because the war launched by the Roman Church, the name under the banner of anti-Islamic, Western European troops participating in the campaign, each equipped with a cross symbol.
1202, the Crusaders start the fourth eastward. Pope Innocent III was initially determined attack target is Egypt. Crusaders no boats to cross the harbor to request the help of The Merchant of Venice. Venice merchant oligarchy in power put forward the conditions of satisfaction as to the occupation of Zadar City, the Crusader leaders to accept the conditions. Zadar is Catholic, so the Pope wrote to the Crusaders scolding meal. At this point, the deposed Byzantine emperor Isaac II's son Alec Alexei to the Pope for help against the usurper Alec Alexei III. So the Crusaders to help the Byzantine recovery "legitimate" the throne as an excuse to enter the Byzantine. June 1203 besieged Constantinople, Alec Alexei III absconded, Isaac II reset. A Alexei help reset the remuneration in order to pay the Crusaders 200,000 silver marks, the implementation of exactions, sparking an uprising of the people of Constantinople, Alec Alexei was hanged. (? V history today?? Com) In April 1204, the Crusaders to launch an attack on Constantinople. The next city compromised. Crusaders in the city of arbitrary killings of unarmed residents, rape women, hold robbery belongings. Most of the residents of the city were massacred dozens of perfect the ancient carved learned destroyed, rich Constantinople library collections ashes, St. Sofia church was ransacked, the looting of many art treasures after this time, and then can not restore vitality. Crusaders infringement of one of the offenses under the East.