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 First Crusade (1095–1099) was the first of a number of crusades that attempted to recapture the Holy Land, called for by Pope Urban II at the Council of Clermont in 1095. 
Urban called for a military expedition to aid the Byzantine Empire, which had recently lost most of Anatolia to the Seljuq Turks. 
The resulting military expedition of primarily Frankish nobles, known as the Princes' Crusade, not only re-captured Anatolia but went on to conquer the Holy Land (the Levant), which had fallen to Islamic expansion as early as the 7th century, and culminated in July 1099 in the re-conquest of Jerusalem and the establishment of the Kingdom of Jerusalem.