The Islamic world is split into two main factions, the Sunni and Shia. The original reason for this split was the disagreement over who should succeed Mohammed after he died. The Sunni favored a hereditary succession while the Shia favored a ‘best man’ succession. The Shia were successful in their desires at first even though a descendant of Mohammed did lead Islam at a later time.

(The Great Courses – Islam)