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.
The Complete History
Ibn al-Athir characterizes the advent of the crusades as an issue of political intrigue and its historical importance in terms of Frankish conquest, as merely one event within a continuous pattern. He attributes the origin to the happenings of 1085-86, when the Franks first invaded Islamic lands in Andalusia, and connects the crusades with the conquest of Sicily in 1091.