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.
First Crusade
The successful Princes' Crusade was preceded by the "people crusade", which was a popular movement 
instigated by Peter the Hermit in the spring of 1096. Mobs of peasants and laymen travelled to Anatolia where they came up against the Turks, on the way attacking populations of Jews in the Rhineland. They were decisively defeated at the Battle of Civetot in October.