In the early days of Islam, a schism split the Islamic community into two groups in the immediate aftermath of the Prophet Muhammad’s death. The great fissuring into Sunnis and Shias would later lead to political and religious tensions between the two blocs, which has flared into outright war from time to time. There have, of course, also been periods of relative calm and mutual understanding between the two sects. But the past 1,400 years have been marked by tension and differences as a result of the ancient schism.

Following the Prophet Muhammad’s death,  the question of his succession arose. When Ansar, or the local inhabitants of Medina, and the Muhajirun, or the followers of the Prophet Muhammad who emigrated with him from Mecca, met in Saqifah pavilion, this question proved to be quite divisive. The Ansar eventually gave up their claim to leadership over the Islamic community, but tensions again flared when the Muhajirun raised the question of a hereditary succession. Some Muslims believed that the Prophet’s closest companion, Abu Bakr, was his legitimate successor. Others, however, claimed that the role should be given to Ali, the prophet’s cousin and closest kin. What actually happened at the Saqifah has been highly contested among Muslim historians, particularly in relation to Ali’s stance and his allegiance to Abu Bakr. In general, members of the Sunni sect downplay the importance of Saqifah meeting, whereas Shiites tend to see the allegiance to Abu Bakr that emerged from the meeting as a betrayal of the Prophet.

Ultimately, Abu Bakr became the Prophet’s first successor and was followed by Umar and Uthman, who were both assassinated. Following Utham’s reign, Ali seized power in 656 AD. His five-year reign was turbulent and marked by war. Ali subdued the revolt mounted by Aisha, the wife of the prophet and the mother of believers, according to Sunni tradition, fought Kharijites and fended off Muawiyah’s ambitions and attempts to seize control of the community. Ali was assassinated in 661 AD by the Kharijites.

Following Ali’s death, his elder son Hasan ibn Ali claimed the caliphate, which brought him into armed conflict with Muawiyah, who had also claimed the caliphate. Following  the Battle of Siffin, the two men signed the Hasan-Muawiyah Treaty, which stipulated Hasan’s allegiance to Muawiyah in return for Muawiyah’s vow to name no successor to succeed him and let the Muslim community select a successor by consensus.

Before Hasan ceded the caliphate to Muawiyah, they exchanged several letters in which Hasan called on his rival to surrender. In these letters, Hasan made clear the way in which both he and his father had understood the question of succession, which would become the basic argument advanced by the Shiites. “If Quraysh [a tribe from Mecca] could claim the leadership over the Ansar on the grounds that the Prophet belonged to Quraysh, then the members of his family, who were the nearest to him in every respect, were better qualified for the leadership of the community,” said Hasan.

In 670 AD, Hasan’s wife poisoned him as a result of Muawiyah’s incitement, though not all historical resources agree on this point. Muawiyah named his son, Yazid Ibn Muawiyah, as his successor, in contravention of the treaty he had signed with Hasan. Naturally, this made him the first suspect in Hasan’s killing.

In 680, Muawiyah died having appointed his son as caliph. Husayn Ibn Ali, the Prophet’s grandson and son of Ali, refused to pay allegiance to Yazid, considering his succession a breach of the Hasan-Muawiyah Treaty.

When Husayn set off for Kufa, his caravan was ambushed by Yazid’s army in the Karbala region. Husayn’s caravan, which consisted of some 70 of his followers and family members, was outnumbered by their enemies. Yazid’s army massacred the travellers and beheaded Husayn. Ever since, Karbala has been considered a symbol of Shia martyrdom and an important site for their community.

The Umayyad Caliphate, and after it the Abbasid Caliphate, waged war against the Shiites. However, the Shiites managed to establish states of their own. The main Shiite states in history were the Fatimid Caliphate and Safavid dynasty. Today, Khomeini’s Iran bears the torch for the Shiite community.

Clearly, the schism between the Sunnis and Shiites has less to do with religious matters than with politics. Yet, during the course of the 1,400-year conflict, the two groups began to accumulate divergent interpretations and rituals.

The Shiites believe the leader of their community, called the Imam, to be sinless by nature. Furthermore, Shia Imams trace their bloodlines back to the prophet and, as such, are the only divinely qualified leaders empowered to handle matters of faith and to define the Shari’a, or Islamic law, that guides the Muslim community. A group with in the Shia sect, called “Twelvers” believe that twelve Imams have been chosen by divine will. The first of these twelve was Ali, and the last one will be known as the  Mahdi, or Messiah, and will single-handedly redeem Islam and rule until the final Day of Judgment.

Sunnis, on the other hand, disregard the importance of bloodlines in both political and religious leadership.

Although Shiites and Sunnis agree upon the most basic pillars of Islam and on the role of the Quran and the hadith, or the words and deeds of the prophet, they disagree on the credibility of certain passages from the hadith and some of the  reported traditions concerning his spiritual life.

Another ritual difference between Sunnis and Shiites is the way they celebrate Ashura, the tenth day of Muharram in the Islamic calendar. In Ashura, the Shiites commemorate the Karbala massacre and mourn Husayn. Sunnis, on the other hand, do not engage in any mourning ceremony, although they also denounce the killing of Husayn.

In fact, this long lasting division between the two major poles of Islam seems far more a political schism than a religious one.