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.
Role of Christianity in civilization
In 1095, Pope Urban II called for a Crusade to re-take the Holy Land from Muslim rule.
"After Mohammad's death in 632, a series of caliphs (successors) waged energetic jihad against neighboring people's. Palestine, Syria, Persia, and Egypt—once the most heavily Christian areas in the world—quickly succumbed. By the eighth century, Muslim armies had conquered all of Christian North Africa and Spain and were moving into France. In 732, at the Battle of Tours, Charles Martel defeated Muslim invaders and sent them back into Spain. By the eleventh century, the Seljuk Turks had conquered Asia Minor (modern Turkey), which had been Christian since the time of St. Paul. The holdings of the old Roman Empire, known to modern historians as the Byzantine Empire, were reduced to little more than Greece. In desperation, the emperor in Constantinople sent word to the Christians of western Europe asking them to aid their brothers and sisters in the East." This was the first crusade, but the "Colossus of the Medieval world was Islam, not Christendom" and despite initial success, the conflicts, which lasted four centuries, ultimately ended in a failure to retake the "Holy Land" and only barely prevented the taking of the European continent.