Starting with a question of leadership
The Prophet Muhammad died in 632, and the community faced an urgent question: who should lead it? A group of leading companions chose Abu Bakr as the first caliph, meaning successor. He was followed by Umar, Uthman and then Ali, the Prophet’s cousin and son-in-law. This is the order accepted by the majority, who became known as Sunnis, a name drawn from the phrase for the people of the tradition (sunna) and the community.
Others held that Muhammad had designated Ali, or that leadership belonged by right to Ali and to descendants of the Prophet through his daughter Fatima. These supporters were called the party of Ali, shi’at Ali, which is the origin of the term Shia. In the earliest years this was a disagreement about politics and legitimacy, not two fully formed religions. The terms and the boundaries hardened gradually over generations, and historians note that early sources use them less sharply than later ones.
Civil war and Karbala
Ali became caliph in 656 after Uthman was murdered, and his rule was marked by the first major civil war, called the first fitna. He was challenged by Muawiya, governor of Syria, and the two sides met in an inconclusive battle at Siffin. Ali was assassinated in 661 by a member of a group called the Kharijites, who had broken from both sides, and Muawiya founded the Umayyad dynasty.
The next defining event came in 680. Ali’s younger son Husayn, who had refused to accept the new Umayyad ruler Yazid, was killed with a small band of followers at Karbala in Iraq. The event became central to Shia devotion, remembered each year with mourning rituals, especially on the day of Ashura. Sunni Muslims also revere Husayn as the Prophet’s grandson, but the memory of Karbala plays a different role in religious life.
What the groups share
It is worth stressing that the two branches agree on core matters: belief in one God, the prophethood of Muhammad, the Quran, and the main acts of worship such as prayer, fasting in Ramadan, charity and pilgrimage to Mecca. The differences concern leadership, religious authority and some points of law and ritual. In practice, Sunni and Shia neighbors often lived together without conflict, and many ordinary people probably did not consider themselves defined by the divide.
Shia branches
Shia Islam developed several branches, mainly through disputes over which descendant of Ali should be the imam, the divinely guided leader.
- Twelver Shia follow a line of twelve imams, the last of whom is believed to be in hiding and to be expected to return. This group became the largest Shia community, and their formal doctrine took shape over the ninth and tenth centuries.
- Ismailis followed a different line after the sixth imam, supporting Ismail and his descendants. The Fatimid caliphate in Egypt was an Ismaili state, and a group of Ismailis known as Nizaris formed in the late eleventh century, producing legends about the so-called Assassins, many of which come from hostile sources.
- Zaydis accepted a more activist model of leadership and a wider pool of candidates among Ali’s descendants. They established a state in the highlands of Yemen, which lasted in some form for many centuries.
Sunni consolidation
Sunni Islam also became more defined, particularly through the development of law and theology. Four main schools of law emerged, named after Abu Hanifa, Malik, al-Shafi’i and Ibn Hanbal. They differed in method and details but regarded each other as legitimate. In theology, scholars debated how far reason could be used to interpret scripture, and over time schools of thought such as the Ash’arites gained wide acceptance.
The Abbasid caliphs were Sunni in their later alignment, but earlier rulers often tried to win support from various groups. The Abbasids themselves came to power with the help of people who wanted a ruler from the Prophet’s family, and then turned against more radical Shia claimants.
Competing states
The divide took on a sharper political edge in the tenth and eleventh centuries. The Buyids, a Shia dynasty, controlled Baghdad from 945 yet left the Sunni Abbasid caliph in place as a figurehead, presumably because he was useful as a source of legitimacy. In Egypt the Fatimids, as Ismailis, claimed the caliphate for themselves and sent missionaries to win followers elsewhere, which Sunni rulers saw as a threat.
The Seljuk Turks, who were Sunni, took Baghdad in 1055 and presented themselves as protectors of the caliph. Their vizier Nizam al-Mulk supported a network of colleges known as the Nizamiyya, which helped teach Sunni law and doctrine and encouraged a Sunni revival. The rivalry between Cairo and Baghdad was one reason the Muslim world responded to the Crusades with so little unity at first when the new Crusader states appeared on the coast. Saladin’s replacement of the Fatimids with Sunni rule in Egypt in 1171 was a significant turning point for the region.
After the Mongol conquests of the thirteenth century, patronage shifted again, and later rulers in Iran favored one branch or the other at different times.
Avoiding simple stories
It is tempting to read medieval history as a straight line of Sunni-Shia conflict. That reading flattens a complex past. Political, ethnic, tribal and economic motives shaped many wars, and Muslims of both branches frequently fought allies and enemies on other grounds. Periods of rivalry existed, as did periods of cooperation and ordinary coexistence. Many cities had mixed communities, and scholars sometimes studied under teachers of different traditions.
The boundaries were also not static. Beliefs shifted, groups split, and new communities formed. What looks like a fixed division today was shaped by centuries of debate, state power and memory.