How do I understand the Crusades?

hi everyone, my familiy is of croatian catholic descent, but since my father is a socialist, I never got a chance to get a proper religious education, so I decided to study it for myself. My biggest enigma so far is if the crusades were according to the teaching of jesus. I know the old testament has many violent (about war) teachings, but as far as I realise, old test. doesn’t apply now (or does it?). if we are to follow the teachings of jesus to love our enemies, turn the other cheek and not resist evil (im confused by these verses as well), than how were the crusades justified and were they sin in the eyes of god. or we are obliged to follow some rules of the old testament and see the new as a spiritual guidance only? Seeing what happens in europe now may justify a new crusade, but would it be an offense to teachings of Jesus? I would appreciate if someone could help me end my confusion

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The Crusades/Inquisition from a pro-Catholic POV

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