Dear Basel,
The history of the Crusades is rather long and complex, but essentially it was a response to Islamic aggression and therefore was a defensive action, compatible with just war theory.
See what the Catechism and St. Thomas Aquinas say on this.
There are many books to consider, but one I am reading now, and very worth the read, is God's Battalions, by Rodney Stark (The Case for the Crusades). I think you could find the answers you are looking for there; find it on Amazon.
Peace,
Bob Kirby
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