Explanation: The Christian Crusades were kicked off by the Byzantine Empire asking the Western Europeans for help against Muslim opponents. However, as the Western Europeans were a slightly different kind of Christian (Catholic) than the Byzantines (Orthodox), this meant that the time-hallowed tradition of treating heretics worse than nonbelievers HAD to rear its head in their interactions. While the (Catholic, Western European) Crusaders were usually the offenders, the (Orthodox, Balkan and Anatolian) Byzantines were certainly not short of backstabbing plots themselves.

In the end, the Crusades were a failure over the long-term for many reasons… but one suspects that the Crusaders taking chunks out of the Byzantines and engaging in a little light coup assistance when the mood struck them, and the Byzantines cutting supplies and abandoning Crusader armies to certain doom when it suited their political aims, did not help matters.