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.
Sectarian violence
Although the First Crusade was initially launched in response to an appeal from Byzantine Emperor Alexios I Komnenos for help in repelling the invading Seljuq Turks from Anatolia, one of the lasting legacies of the Crusades was to "further separate the Eastern and Western branches of Christianity from each other."