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.
List of sources for the Crusades
Greek Orthodox historians:
Michael Psellos, Anna Komnene, John Kinnamos, Niketas Choniates, Nicephorus Gregoras, George Acropolites.
The Eastern Orthodox view of the Crusades is dominated by the sack of Constantinople and the establishment of the Frankokratia in the Fourth Crusade in 1204. While the First Crusade was declared upon the request of the Byzantine Emperor, and concluded in triumphal success, the Crusades as a whole 
resulted in the terminal decline of the Byzantine Empire. Although the Byzantine Emperors were restored in 1260,
the empire could never fully recover until eventually, weakened by civil wars, it fell to the Ottomans in 1453.