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.
Crusader states
In the Baltic region, the indigenous tribes in the Middle Ages at first staunchly refused Christianity. In 1193, Pope Celestine III urged Christians to have a crusade against the heathens which included the Old Prussians, the Lithuanians and other tribes inhabiting Estonia, Latvia and East Prussia. This period of warfare is called the Northern Crusades.