From where SueZVudu ended off : the Fourth Caliph (Khalîf) rose to power in spite of great protest as people saw him as more scholar and poet than a political leader and his supporters the Shî'a (partisans) were suddenly pitched in civil war with the more orthodox Sunni ('Followers of the Path'). This led the schism and splinter of Islam, and those two groups quickly divided into still smaller sectarian groups. The Shî'a believed the new Imam was himself divine, and that the soul of the Prophet Muhammed passed through each (much like the Pope of the Catholic Church is considered infallible). One of the key tools in the growth of Isma'îlianism (a branch of the Shî'a) who followed the line of the seventh Imam (and so were also called 'Seveners') was the use of their devotees (fid'is) to convert or eliminate opponents as a final act of dedication to the grand master. The Syrian term for the followers was the hashshashin, or hashish takers, and when the Crusaders stumbled into their territory in the Levant, that term was taken as assassin, who were soon feared throughout the Middle East and beyond for their silence and severity.
    In order to survive the reprisals of their sworn enemies, the Isma'îlî began the practice of taqîya (the ritual concealment of belief as religious practice)1 which by the 10th c. was considered obligatory along most trade routes, pilgram paths and cities in the Arab world. As Burton later explains it was 'the systematic concealment of anything that concerns their faith, history and customs', and this was practiced to the point of art among the Sunnis. However the peace of this polite compromise of concealment did not last. By the 13th c., the Holoku Khan, a Moslem mongol prince, declared open war upon the Shî'a heresy and his armies overran the Fida'is fortresses in Persia. The Central Isma'îlî stronghold at Alamüt was captured and some 12,000 slaughtered. The Seveners in Syria were likewise hunted and exterminated out of fear of their unseen practices and revenge. The struggle between these two groups within Islam continues unabated today.2 The Iran - Iraq War of the 1980s were fought along these same lines of doctrine (though again, religious rhetoric often conceals a more profane, political motive, as with every conflict).
1. The challenge of fundamentalism : political Islam and the new world disorder / Bassam Tibi Berkeley : University of California Press, 1998.
2. Muslim kingship : power and the sacred in Muslim, Christian and pagan polities / Aziz Al-Azmeh. London ; New York : I.B. Tauris, 1997.

1 Would that the frighteningly devout fundamentalist movements today (of all creeds) had the foresight, grace and general social manners to do the same, and not be so painfully obvious. Clearly 10th century Arabia was a uniquely civilized time.
2 Like the Catholics & Protestants in Northern Ireland, or the orthodox and liberal Sikhs in Southeast Asia, these religious splits begin largely over politics rather than doctrine, and only acquire a theological tone after some time (usually as a pretext to further violence).