"With enormous energy, the warriors of Islam struck out against the Christians shortly after Mohammed's death. They were extremely successful. Palestine, Syria, and Egyptâ€once the most heavily Christian areas in the worldâ€quickly succumbed. By the eighth century, Muslim armies had conquered all of Christian North Africa and Spain. In the eleventh century, the Seljuk Turks conquered Asia Minor (modern Turkey), which had been Christian since the time of St. Paul. The old Roman Empire, known to modern historians as the Byzantine Empire, was 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."
That is what gave birth to the Crusades