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.
History of Islam
In the Third Crusade armies from Europe failed to recapture Jerusalem, though Crusader states lingered for several decades, and other crusades followed. The Christian Reconquista continued in Al-Andalus, and was eventually completed with the fall of Granada in 1492. During the low period of the Crusades, the Fourth Crusade was diverted from the Levant and instead took Constantinople, leaving the Eastern Roman Empire (now the Byzantine Empire) further weakened in their long struggle against the Turkish peoples to the east. However, the crusaders did manage to damage Islamic caliphates; according to William of Malmesbury, preventing them from further expansion into Christendom and being targets of the Mamluks and the Mongols.