The Crusaders in the Middle Ages were wars of aggression against countries in the Near East by the Holy See, knights in Western Europe and feudal lords in eastern Mediterranean countries. This war lasted nearly two centuries from 1096- 1270.

Why is it called "crusade"? Because the initiator of the war was the Roman church, under the banner of anti-Islam, everyone in the western European army who participated in the war was equipped with a cross.

1202, the Crusaders began their fourth eastward invasion. Pope Innocent III's initial target was Egypt. But the Crusaders didn't have any ships to cross the sea, so they turned to Venetian merchants for help. The Venetian oligarch, a businessman in power, offered to occupy zadar as compensation condition, and the leader of the Crusader accepted this condition. Zadar is a Catholic, so the Pope wrote a letter to the Crusaders. Just then, Alexei, the son of deposed Byzantine emperor Isaac II Angelos, turned to the Pope for help and opposed the usurper Alexei III. So the Crusaders marched into Byzantium on the pretext of helping Byzantium restore its "legitimate" throne. 1June 2003, Constantinople was surrounded, Alexei III fled, and Isaac II Angelos was restored. In order to pay the 200,000 silver marks that the Crusaders helped to reset, Alexei levied taxes, which aroused the uprising of the people in Constantinople and Alexei was hanged.

1In April 2004, the Crusaders launched an attack on Constantinople. The next day, the city fell. Crusaders arbitrarily slaughtered unarmed residents, raped women and robbed property in the city. Most of the residents in the city were massacred, dozens of perfect ancient sculptures were destroyed, the rich collection of books in Constantinople Library was reduced to ashes, and the church of St. Sofia was looted, and many art treasures could not be recovered after this looting. This is one of the crimes committed by the Crusaders.