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.
Crusades
The Crusades consolidated the papal leadership of the Latin Church, reinforcing the link between Western Christendom, feudalism, and militarism and increased the tolerance of the clergy to violence. The growth of the system of indulgences became a catalyst for the Protestant Reformation in the early 16thcentury. The Crusades also had a role in the creation and institutionalisation of the military and the Dominican orders as well as the Medieval Inquisition.