Shi’a Islam is the only major schism in Islam. It is not a recent schism, however, for it dates back right to the foundations of Islam. Shi'ite historians believe that Shi'ism began shortly after the death of Muhammad, when the Caliphate, or secular leadership of Islam, was handed to Muhammad's father-in-law, Abu Bakr, rather than 'Ali, Muhammad's chosen successor. The Muslims who supported 'Ali called themselves the "Partisans of 'Ali" (Shi’a 'Ali ); these supporters, who were only four in number, are the root of Shi’a Islam. Western and Sunni historians date Shi'ism as a religion to the death of Husayn, the grandson of Muhammad, in the battle of Karbala. The celebration of this martyrdom by the Shi’a 'Ali represents for these historians the first clear instance of separate religious practice.
However that may be, Shi’a Islam is a crucial part of the Islamic tapestry throughout the history of Islam. World history textbooks tend to blissfully ignore the Shi'ite adventure through history, but the minority Shi'ites have played a determining role in Islamic and world history. Most recently, Shi'ism has given rise to a new Islamic political theory called velayat-i faqih , or "rule by jurisprudence" (Westerners call it "Islamic Republicanism"). Velayat-i faqih is perhaps the most important Islamic innovation of our century; it may be very possible that by the end of the century, the bulk of the Islamic world will be practicing it in one form or another. Or maybe not. You're sitting at one of these great moments in history, in which an entirely new way of conducting human culture and human business has been invented in your lifetime. The Iranian Revolution of 1979 may turn out to be the most significant event in Islamic history for the last five centuries. Or it may not. The Revolution and the political theory it spawned, however, are continuing evidence of the crucial role that Shi'ism plays in the global community.
'Ali
The foundational figure in Shi’a history is 'Ali, the son-in-law and cousin of Muhammad. After the death of Muhammad, rival claims were put forth for the caliphate which was the office that was the supreme secular authority of Islam. In Shi’a history, Muhammad designated 'Ali as his successor, so that all the others who served in this capacity were illegitimate. The "Partisans of 'Ali," Shi’a 'Ali in the struggle to get 'Ali in the Caliphate and in the civil war that broke out when 'Ali was finally named Caliph gave the name to the religious schism that divided the Islamic world from the very beginning. Eventually the Shi'ites would develop a religious doctrine that differs in fundamental respects from orthodox, or Sunni Islam. Nevertheless, at the cornerstone of Shi’a history is the figure of 'Ali and his persecution by the illegitimate caliphs.
Upon Muhammed's death, a hastily collected group of prominent Muslim leaders elected Muhammed's father in law, Abu Bakr, to be the secular head of Islam. However, 'Ali, Muhammed's son-in-law and cousin, was not part of this committee nor were other members of Muhammed's immediate family, and many believed that Muhammed had designated 'Ali as a successor, for the Traditions had Muhammed naming him as both his brother and his successor. 'Ali had been raised with Muhammed and was the second person (after Muhammed's wife Khadija) to recognize Muhammed's role as a prophet; he was the first of Muhammed's tribe, the Quraysh, to declare himself an apostle. But the Meccan and Medinan leaders, with no members of Muhammed's house present, gave their allegiance to Abu Bakr as Caliph, or Successor to Muhammad and supreme head of Islam, and attempted through force of arms to coerce 'Ali into acknowledging Abu Bakr as well. After the caliphates of 'Umar and 'Uthman, 'Ali became caliph n 656. The Umayyads who ruled the various governments, however, revolted and established the Umayyad caliphate.
From this point onwards, authority was divided in the Islamic world. The Umayyads continued as caliphs; but there now existed in Iraq a separate Islamic community that did not recognize the authority of the Umayyad Caliphs. Rather they recognized only the successors to 'Ali as authorities, and they gave these successors the title Imam, or spiritual leader of Islam, both to differentiate their leaders from the more worldly and secular Umayyads. They called themselves Shi’a 'Ali, or "The Partisans of 'Ali," and are called by historians, 'Alids.
Husayn
In Shi'ite history, 'Ali is the first Imam and was followed by a grand total of eleven Imams, who passed the Imamate down to their sons in hereditary succession. However, the most important Imam of Shi’a was Husayn, whose martyrdom at Karbala is the most important event in the Shi’a experience of history. Husayn was killed by Yazid, the second Umayyad caliph, because of the growing threat the 'Alids posed to caliphal power. The successful massacre of Husayn and his followers was in part due to the failure of Shi'ites to rally to their Imam—so the martyrdom of Husayn represented to Shi'ites both the illegitimacy of Islamic authority and Shi'ite failure to bring about legitimate Islamic rule.
The Imamate
From the abdication of 'Ali onwards, authority was divided in the Islamic world. The Umayyads continued to pass the Caliphate down through the ages among their family; but their now existed in Iraq a separate Islamic community that did not recognize the authority of the Umayyad Caliphs. Rather they recognized only the successors to 'Ali as authorities, and they gave these successors the title Imam. A grand total of eleven Imams succeeded 'Ali (ten in non-Shi'ite histories), passing the Imamate down to their sons in hereditary succession. However, the eleventh Imam (the tenth Imam to succeed 'Ali), Hasan al-Askari, died without a son, and the Shi'ites were thrown into disarray. Shi’a Islam divided into several different sects, the most important of which was the Qat'iyya ("those who are certain"). The Qat'iyya believed that Hasan al-Askari did indeed have a son, Muhammed al-Mahdi; one of the Qat'iyya sects believed that Muhammed al-Mahdi, the twelfth Imam, had hidden himself and remained in hiding. This sect was called Ithna-'Ashari (Twelver) or Imami (Imam) Shi’a, and was the form of Shi’a that eventually came to exclusively represent Shi'ism. When you say Shi’a, you really mean Twelver Shi’a.
The Imamate is the central aspect of Shi'ite Islam and what principally distinguishes it from orthodox or Sunni Islam. In all other respects, Shi’a Islam is virtually identical with Sunni practice. In the Shi’a concept of the Imam, a central belief is that at no time in human history has the world been bereft of an Imam. The Imam is a gift by God to humanity; he serves as both a guide (Hadi) to humans, a Proof of God (Hujjat Allah ) and a Sign of God (Ayyat Allah ). The Imams span history from Adam, the first Imam, to the present day; Muhammad himself was an Imam. The Imams, according to Shi'ites, were a light created before the creation—this light was the instrument of creation and is embodied in each Imam. The Imam has secret knowledge of God and creation; the most important of these secrets is "The Greatest Name of God." The Imams are designated or appointed (mansus ) by God and they are free from all sin or fault (ma'shum ); therfore, they are the most perfect of humans (afdal an-nas ). But above all, the Imam is the one who teaches human beings the mystical truths of the universe. The Shi'ites believe that the Imam that the esoteric, mystical aspects of God are transmitted to human beings.
The son of Hasan al-Askari, however, hid himself away from men in order to preserve his life. This last Imam, the twelfth Imam, has been "hidden" or "occulted" and still is alive today on earth. "The Guided One," as he is called, is awaiting the time when he will return, guide the world, and restore Shi’a to its proper place as the universal religion of God.