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 papacy
Long-standing divisions between East and West also came to a head in the East–West Schism and the Crusades. The first seven Ecumenical Councils had been attended by both Western and Eastern prelates, but growing doctrinal, theological, linguistic, political and geographic differences finally resulted in mutual denunciations and excommunications. Pope Urban II (1088–99) speech at the Council of Clermont in 1095 became the rallying cry of the First Crusade.