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From a historic viewpoint as recorded, with Muhammad's death in AD 632, disagreement broke out over who should succeed him as leader of the Muslim community. Umar (Umar ibn al-Khattab), a prominent companion of Muhammad, nominated Abu Bakr. Others added their support and Abu Bakr was made the first caliph. This choice was disputed by some of Muhammad's companions, who held that Ali (Ali ibn Abi Talib), his cousin and son-in-law, had been designated his successor.[1] None of Muhammad's sons survived into adulthood, therefore direct hereditary succession was never an option. Later, during the First Fitna and the Second Fitna the community divided into several sects and groups, each of which had its own idea about successorship. Finally, after the Rashidun caliphate turned into Monarchies and Sultanates, while in most of the areas during Muslim history Sunnis have held power and Shiites have emerged as their opposition.From a religious viewpoint, Muslims later split into two groups, Sunni and Shia. Sunnis assert that even though Muhammad never appointed a successor, Abu Bakr was elected first caliph by the Muslim community. The Sunnis recognize the first four caliphs as Muhammad's rightful successors. Shias believe that Muhammad explicitly named his successor Ali at Ghadir Khumm and Muslim leadership belonged to him who had been determined by divine order.
That is from Wikipedia.. And that's what all this infighting is about. Iran is Shiite. . ISIS is Sunni.
