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.
First Crusade
The causes of the Crusades in general, and particularly of the First Crusade, is widely debated among historians. 
While the relative weight or importance of the various factors may be the subject of ongoing disputes, it is clear that the First Crusade came about from a combination of factors in both Europe and the Near East.
Its origin is linked both with the political situation in Catholic Christendom, including the political and social situation in 11th-century Europe, the rise of a reform movement within the papacy, as well as the military's and religious confrontation of Christianity and Islam in the East.