In any large scale war were Christians and/or Muslims are killing each other, you will usually find the bankers (jews) hand in instigating it.

The crusades are no different.

Interesting facts:

One of the key instigators of the crusades were Peter the Hermit, Peter came from the Orval Abbey in Stenay in the Ardennes.

Who funded that Abbey? The crypto jewish Merovingian family of Lorraine!

Guess who was the first king of Jerusalem during the Crusades time? Godfrey of Bouillon who was taught by none other than Peter the Hermit.

He was put as king of Jerusalem due to his jewish lineage as king of the jews. He was also a descendant of William of Gellone.

Also worth noting how the Venetians (crypto jews who came from Spain/Portugal), sponsored and profited from the crusades, Webster Tarpley's work is valuable on this matter.