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 Islam
Beginning in the 8th century, the Iberian Christian kingdoms had begun the Reconquista aimed at retaking Al-Andalus from the Moors. In 1095, Pope Urban II, inspired by the conquests in Spain by Christian forces and implored by the eastern Roman emperor to help defend Christianity in the East, called for the First Crusade from Western Europe which captured Edessa, Antioch, County of Tripoli and Jerusalem.