The division between Sunni and Shia Moslems, which has become so prominent in Iraq, goes back to the earliest days of islam,and has received far too little attention from our political leaders.
The success of the First Crusade was very largely due to this division. Jerusalem had recently been taken over by the Egyptians, who were ruled by the Fatimids, a Shia dynasty. The Egyptians and the Turks (who were Sunni) hated each other at least as much as they both hated the crusaders, and were never able to co-operate properly against them. But once Saladin had conquered Egypt and restored it to the Sunni faith, the crusades were doomed.
Saladin was a Kurd, from a family of mercenary soldiers. His battles against Richard the Lion-Heart won him the reputation in Europe as a romantic chivalrous warrior. Dante in his "Inferno" places Saladin, uniquely, in the circle of "Virtuous pagans", alongside the ancient Greek and Roman philosophers. But as far as I am aware, Saladin is given far less status in Moslem tradition.