Shia-Sunni sectarian conflicts have been a feature over most of Muslim history, and they have closely linked to the competition for power. It was this that led Syed Amir Ali (writer on Islamic history and society) to remark in his book, The Spirit of Islam, "Alas! That the religion of humanity and universal brotherhood should not have escaped the internecine strife and discord; that the faith which was to bring peace and rest to the distracted world should itself be torn to pieces by angry passions and the lust of power".
Shortly after the death of the Prophet of Islam (PBUH), the early Muslim society was divided on the question of succession to the position of leadership of the community. A small group believed that the function must remain within the family of the Prophet, and backed 'Ali', whom they believed to have been designated for this role by appointment (ta'yin) and testament (nass). They believed that the spiritual heritage bequeathed by Mohammad (PBUH) devolved on Ali and his lineal descendants. Hence, they repudiated the authority of the jama'at (the people) to elect their leader. They became known as his 'partisans' (shi'ah). On the other hand, the majority agreed on Abu Bakr as the leader on the assumption that the Prophet left no instruction on this matter. They gained the name 'The People of Prophetic Tradition and consensus of opinion' (ahl al-sunnah wa'l-jama'ah).
Besides the political dimension, there also existed a difference of opinion about the merits and functions of the successor to the Prophet. Sunni Islam considered the Khalifah to be a guardian of the shariah in the community, while Shi'ism saw in the 'successor' a spiritual function connected with the esoteric interpretation of the revelation and the inheritance to the Prophet's 'hidden' teachings. In contrast to the Sunnis, the institution of Imamate is fundamental to the Shias. The Imam, besides being a descendant of the Prophet, must possess certain qualities — he must be ma'sum or sinless, bear the purest and most unsullied character, and must be distinguished above all other men for truth and purity. On the other hand, the Sunnis believe that the Imamate is not restricted to the family of Mohammad (PBUH), that the Imam need not be irreproachable (ma'sum) in his life, and nor need he be the most excellent or eminent being of his time. So long as he is free, adult, sane, and possessed of the capacity to attend to the ordinary affairs of state, he is qualified for election.
In general, the Sunnis continued to support the established authority of Ummayads and Abbasides, though the later Sunni jurists accepted only the first four caliphs as full embodiments of the ideal of caliphate. For their part, various Shia groups continued to challenge the legitimacy of different caliphates for the most part of Muslim history. The Shias, however, enjoyed political power in the fourth century under the Buyides, who controlled all of Persia and wielded power in Baghdad, and later under the Fatimids in Egypt. Amongst the Shias, the Itna 'Asharis, followers of the twelve saintly Imams, reprehended the use of force, and maintained an attitude of complete withdrawal from temporal power until Shah Ismail, the great Safavi monarch, made Ithna 'Ashari Shi'ism the state religion of Persia. Under Shah Ismail a vigorous campaign was launched to convert the majority Sunni population to Shi'ism.