Prophet Mohammad died without appointing a successor to lead the Muslim community, and disputes arose over who should lead the new and growing faith after his death..
Some followers believed that a new leader should be chosen by consensus; others thought that only the Prophet's descendants should become caliph. The title passed to the trusted aide, Abu Bakr, though some thought it should have gone to Ali, the Prophet's cousin and son-in-law. Ali eventually became the caliph after Abu Bakr's two successors were assassinated.
After assassination of Ali at the mosque at Kufa in Iraq, his sons Hasan and then Hussein, claimed the title. But Hussein and many of his relatives were massacred in Karbala in 680 A.D.
His martyrdom became a central tenet to those who believed that Ali should have succeeded the Prophet. (It is mourned every year during the month of Muharram). The followers became known as Shias, or followers of Ali. The Sunnis regard the first three caliphs before Ali as rightly guided and themselves as the true adherents to the Sunnah, or the Prophet's tradition. Sunni rulers embarked on sweeping conquests that extended the caliphate into North Africa and Europe. The last caliphate ended with the fall of the Ottoman Empire after World War-I.
The Sunni and Shia sects encompass a wide spectrum of doctrine, opinion and schools of thought. The branches are in agreement on many aspects of Islam, but there are considerable disagreements within each. Both branches include worshippers who run the gamut from secular to fundamentalist.
Shias consider Ali and the leaders who came after him as Imams. Most believe in a line of 12 Imams, the last of whom, a boy, is believed to have vanished in the ninth century in Iraq after his father was murdered. Shias known as 'Twelvers' anticipate his return as the Mahdi.
Sunnis emphasise God's power in the material world, sometimes including the public and political realm, while Shias value martyrdom and sacrifice.