Military history of the Crusader states
The Seljuk–Crusader War began when the First Crusade wrested territory from the Seljuk Turks during the Siege of Nicaea in 1097 and lasted until 1128 when Zengi became atabeg of Aleppo. At the latter date, the chief threat to the Crusaders from the east and north became the Zengids. The conflict was generally fought between European Crusaders and the Seljuk Turks and their vassals. The Muslim Syrian emirates occasionally allied themselves with the Christians against rival states.
History of the Catholic Church
Pope Urban II launched the First Crusade in 1095 when he received an appeal from Byzantine emperor Alexius I to help ward off a Turkish invasion. Urban further believed that a Crusade might help bring about reconciliation with Eastern Christianity. Fueled by reports of Muslim atrocities against Christians, the series of military campaigns known as the Crusades began in 1096. They were intended to return the Holy Land to Christian control. The goal was not permanently realized, and episodes of brutality committed by the armies of both sides left a legacy of mutual distrust between Muslims and Western and Eastern Christians. The sack of Constantinople during the Fourth Crusade left Eastern Christians embittered, despite the fact that Pope Innocent III had expressly forbidden any such attack. In 2001, Pope John Paul II apologized to the Orthodox Christians for the sins of Catholics including the sacking of Constantinople in 1204.