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.
Jewish history
Sermonical messages to avenge the death of Jesus encouraged Christians to participate in the Crusades. The twelfth century Jewish narration from R. Solomon ben Samson records that crusaders en route to the Holy Land decided that before combating the Ishmaelites they would massacre the Jews residing in their midst to avenge the crucifixion of Christ. The massacres began at Rouen and Jewish communities in Rhine Valley were seriously affected.