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.
Siege of Acre (1291)
The 14th century saw some other crusades organized, but these enterprises differed in many ways from the 11th- and 12th-century expeditions which are properly called Crusades. The crusades of the 14th century aimed not at the recapture of Jerusalem and the Christian shrines of the Holy Land, but rather at checking the advance of the Ottoman Turks into Europe. While many of the crusaders in these 14th-century undertakings looked upon the defeat of the Ottomans as a preliminary to the ultimate recapture of the Holy Land, none of the later crusades attempted any direct attack upon Palestine or Syria.