(I said elsewhere:) I'm in no way trying to excuse the Crusades, which are inexcusable. But I often hear people talking as if the Crusaders went out to fight followers of a different religion, and this is a misunderstanding. The Crusaders went out, as they thought, to fight followers of the *same* religion: they thought Islam was a heretical form of Christianity rather than a separate belief system. That's why the Pope thought he had jurisdiction to order the crusade in the first place. As I said, that doesn't excuse anything they did, but nobody can begin to understand someone without understanding their motivation.