Bill Warner, PhD: Jihad vs Crusades Take a look. And remember this the next time some ignorant or deceitful Islam apologist brings it up.
Another good piece:
Real History Of The Crusades, The"That is what gave birth to the Crusades. They were not the brainchild of an ambitious pope or rapacious knights but a 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. The Crusades were that defense. "