It was in response to the Byzantine request for aid that the Pope seized on the idea of a campaign to recover Jerusalem from the Saracens. It would be a way of uniting the warring kingdoms of Christendom in a common cause, particularly channeling the aggressive Normans, who were then making their way into the Mediterranean. And as an added bonus, it might result in the Catholic Church in Rome achieving supremacy over the Greek Orthodox church in the East.

For the participants in the Crusades themselves, they were never threatened by Muslim aggression, so the idea that they were participating in a defensive conflict is ridiculous. The leaders of the campaigns were more interested in carving out principalities for themselves in the Levant. If they were really serious about rolling back Islam itself, they would have assisted the Byzantines in completely expelling the Seljuk Turks from Asia Minor. After all, as long as the Seljuk sultanate of Konya remained in central Asia Minor, the Crusader states in the Levant could never really be secure.

Of course, the ultimate irony is that the Crusades may have unintentionally facilitated the rise of the Ottoman Turks and their advance into Europe. By seizing the Byzantine capital of Constantinople in 1204, the Crusaders weakened the one Christian power that had long prevented the Muslims from gaining a foothold in Europe.