The truth about Crusading

In popular Western culture today, the Crusades are generally remembered as a series of holy wars against Islam led by power-hungry popes and fought by religious fanatics. They were the epitome of self-righteousness and intolerance, a black stain on the history of the Catholic Church in particular and Western civilization in general. A breed of proto-imperialists, the crusaders introduced Western aggression to the peaceful Middle East and then deformed the enlightened Muslim culture, leaving it in ruins. For variations on this theme one need not look far. See, for example, the 1995 BBC/ A&E documentary, The Crusades, hosted by Terry Jones, or the 2005 History Channel documentary, or the 2005 epic Ridley Scott film, The Kingdom of Heaven. Indeed, I can think of no popular media portrayal of the Crusades that does not hold in some measure to this view.
Yet, that is not at all the way that Europeans viewed the Crusades when they were happening. Indeed, far from being an offensive attack on the lands of Islam, Western Christians saw the Crusades as defensive reactions to Muslim aggression.

And they had a point. Christians in the eleventh century were not paranoid fanatics. Muslims really were gunning for them. From the time of Mohammed, the Muslim state had expanded by the sword. Traditional Muslim thought divided 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 their states must be destroyed and they must be 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. – from The Crusades Controversy