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.
Sixth Crusade
The Sixth Crusade started in 1228 as an attempt to regain Jerusalem. It began seven years after the failure of the Fifth Crusade and involved very little actual fighting. The diplomatic maneuvering of the Holy Roman Emperor and King of Sicily, Frederick II, resulted in the Kingdom of Jerusalem regaining some control over Jerusalem for much of the ensuing fifteen years (1229–39, 1241–44) as well as over other areas of the Holy Land.