Check out Christ the King, Lord of History by Anne Carroll. It’s a fun fast read of the history of the Catholic Church. It explains the rise and fall of the Crusades.
The crusades in general was a good cause. Islam from the 6th century started taking Christian land by force. Christians were coerced to convert to Islam and soon enough they made their way into Europe. By the time the Crusades were organized many lands and Catholics have already been converted to Islam. All four big waves should have been successful but many mishaps happened, in Constantinople they got sidetracked and got into a political mess. When they got to Jerusalem, they got hysterical and killed Muslims, Christians and Jews. Very bad.
But the main thing I want to point out that the Crusades had many causes, one was a true intent on defending Christendom, many I’m sure wanted chivalrous glory and others wanted to get rich by conquering under the banner of Christianity. Modern historians tend to gloss over the first two reasons and focus only on the conquer and get rich part.
The movie showed the crest of this long episode. It would have a completely different feel if it showed how the Muslims got there in the first place.
Do investigate the whole of Christian history, you’d be surprised that the Church which should have died at least 5 times rise from the ashes. It is nothing but miraculous.