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 Islamic prophet Muhammad founded Islam in the Arabian Peninsula and had united much of Arabia into a single polity by his death in 632. Arab power expanded rapidly in the 7th and 8th centuries largely by military conquest. This influence spread to the north-west Indian subcontinent, across Central Asia, the Middle East, North Africa, southern Italy, the Iberian peninsula and the Pyrenees. Jerusalem was taken from the Byzantine Empire after a siege in 637.