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.
Barons' Crusade
The Barons' Crusade, also called the Crusade of 1239, was in territorial terms the most successful crusade since the First. Called by Pope Gregory IX, the Barons' Crusade broadly spanned from 1234-1241 and embodied the highest point of papal endeavor "to make crusading a universal Christian undertaking." Gregory called for a crusade in France, England, and Hungary with different degrees of success. Although the crusaders did not achieve any glorious military victories, they used diplomacy to successfully play the two warring factions of the Muslim Ayyubid dynasty (As-Salih Ismail in Damascus and As-Salih Ayyub in Egypt) against one another for even more concessions than Frederick II gained during the more well-known Sixth Crusade. For a few years, the Barons' Crusade returned the Kingdom of Jerusalem to its largest size since 1187.