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.
Christian pilgrimage
The crusades were at first a success, the Crusader states, especially the kingdom of Jerusalem, guaranteeing
safe access to the Holy Land for Christian pilgrims during the 12th century, but the enterprise of the crusades was ultimately doomed to failure, and the Holy Land was entirely re-conquered by the Ayyubids by the end of the 13th century.