Medieval scholare Thomas F. Madden describes The Real History of the Crusades:
Christians in the eleventh century were not paranoid fanatics. Muslims really were gunning for them. While Muslims can be peaceful, Islam was born in war and grew the same way. From the time of Mohammed, the means of Muslim expansion was always the sword. Muslim thought divides the world into two spheres, the Abode of Islam and the Abode of War. Christianity — and for that matter any other non-Muslim religion — has no abode. Christians and Jews can be tolerated within a Muslim state under Muslim rule. But, in traditional Islam, Christian and Jewish states must be destroyed and their lands conquered. When Mohammed was waging war against Mecca in the seventh century, Christianity was the dominant religion of power and wealth. As the faith of the Roman Empire, it spanned the entire Mediterranean, including the Middle East, where it was born. The Christian world, therefore, was a prime target for the earliest caliphs, and it would remain so for Muslim leaders for the next thousand years.With enormous energy, the warriors of Islam struck out against the Christians shortly after Mohammed’s death. They were extremely successful. Palestine, Syria, 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. 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.
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When we think about the Middle Ages, it is easy to view Europe in light of what it became rather than what it was. The colossus of the medieval world was Islam, not Christendom. The Crusades are interesting largely because they were an attempt to counter that trend. But in five centuries of crusading, it was only the First Crusade that significantly rolled back the military progress of Islam. It was downhill from there.