I find it interesting that people (typically outside of Christianity) always seem to have this attitude that Christians are supposed to act like doormats.
As has been pointed out, the reasons behind the crusades were varied, but one thing I find consistently overlooked by everyone who criticizes Christianity for the crusades, is the fact of 400 years of continual Muslim aggression. We simply would not have the freedoms we enjoy today had it not been for the Christian church keeping the Islamic world out of Europe. The crusades were an inseparable part of that plan.
The other part of the equation has to do with the "God of the Old Testament." It is obvious from the reading of scripture that in many instances, God commanded war as a direct affront to evil itself, and as a tool of punishment to both Israel and the nations that surrounded her.
Certain factions of Christianity like to engage in pacifism and say things like, "Jesus would 'never' do this, or that...." But in saying so, they neglect that Jesus also claimed to be the God of the Old Testament revealed in human flesh. Thus, while we can clearly see the compassion and love of God revealed in Christ, it is at the same time impossible to separate Him from the actions of the Old Testament God of anger and punishment.
One also has to wonder about the prophecies in the Book of the Revelation. Christ will return and destroy - as in; kill - His enemies and throw them into the lake of fire for all eternity. After all, Armageddon doesn't exactly end in a big group hug, now does it?
While I agree that we are to be generally peaceful, the Bible does limit the action of being peaceful to the extent that such is reasonably possible. (Romans 12:18) Distasteful, or not; Sometimes you just have to get out there and kick somebodies a**!! Besides, how does one (Isaiah 1:17) "Rebuke the oppressor; Defend the fatherless, (and) Plead for the widow." if you don't get out there and rile a few people once in a while?