In 1095 the Byzantine Emperor appealed to the Pope for mercenaries to support his fight with the Muslim Turks who had reduced the once expansive empire to just Greece and coastal Asia Mino--and not threatened the capital of Constantinople. Pope Urban II responded by delivering a sermon that called Europeans to a "military pilgrimage."
Tens of thousands of nobles, knights, and soldiers, as well as untrained peasants—men, women and children—responded, initiating a crusading era that lasted nearly two centuries.
In modern times, it is generally thought that the Crusades were not a good idea.
But the facts of the Crusades are generally less well-known than the myths and legends. And some contemporary historians see the Crusades as a crucial (if less than successful) defensive effort in “response to more than four centuries of conquests in which Muslims had already captured two-thirds of the old Christian world. At some point, Christianity as a faith and a culture had to defend itself or be subsumed by Islam” (Thomas Madden, The Real History of the Crusades).
The Holy Land of relics and shrines
Medieval Chaos in the Christian West
The Split between Western and Eastern Christianity
The Growth of the Islamic Empires
Highlights of the Major Crusades
The Legacy of the Crusades