Sunni–Shia split
succession dispute from 632; Karbala 680· Idea, religion or movement· Middle East & Africa
Well documentedSolid records, physical evidence or scientific consensus.
Islam's main division, which began as a dispute over who should lead the Muslim community after Muhammad's death.
See it on the History map →What happened
In 632 most Muslims accepted Abu Bakr, chosen by leading companions, as the first caliph. A minority believed Muhammad had named his cousin and son-in-law Ali, and that leadership belonged to Ali's family. Ali did become the fourth caliph but was assassinated in 661. In 680 his son Husayn and a small band were killed by an Umayyad army at Karbala in Iraq. Over centuries the two sides developed separate traditions of law, leadership and ritual.
Why it matters
About 85–90% of Muslims today are Sunni and 10–15% Shia; the divide still shapes politics, including rivalry between Shia-led Iran and Sunni-led Saudi Arabia.
How we know
Early Arabic histories, mostly written in the 700s–900s, from both Sunni and Shia viewpoints; Ashura mourning rites for Husayn kept for over a millennium; modern population surveys.
Still debated
Sunni and Shia sources disagree on whether Muhammad named Ali as heir, especially on the meaning of his sermon at Ghadir Khumm in 632.
Connections (2)
▸Islam divided in the Sunni–Shia splitWell documented
The connection
Muhammad died in 632 without a succession plan all accepted. Most followers backed Abu Bakr, chosen by leading companions; the 'party of Ali' (shiat Ali) believed leadership belonged to Ali, Muhammad's cousin and son-in-law, and his descendants. The killing of Ali's son Husayn at Karbala in 680 turned a political dispute into a lasting religious divide.
When and how
Medina, 632; Ali's caliphate and the civil wars of 656–661; the Battle of Karbala in Iraq, October 680.
How we know
Early Arabic histories from both traditions; the yearly Shia mourning of Husayn at Ashura, observed for well over a thousand years.
Debates and open questions
When Sunni and Shia became fully distinct identities is debated; many historians see the line hardening only by the 800s–900s.
▸Sunni–Shia split shaped the Shia identity of the Iranian RevolutionWell documented
The connection
Shia Muslims hold that leadership belonged to the Prophet's family and their imams; Twelvers believe the 12th imam went into hiding in 874, and clerics guide believers in his absence. Iran became Shia under the Safavids from 1501. Khomeini extended that guidance into clerical rule, and protesters used the imagery of Hussein's martyrdom at Karbala against the shah.
When and how
Khomeini set out his theory of 'guardianship of the jurist' in lectures in Najaf, Iraq, in 1970; 1978 protests followed the Shia cycle of 40-day mourning.
How we know
Khomeini's writings, revolutionary slogans and posters invoking Hussein and Karbala, and Iran's 1979 constitution, which makes Twelver Shia Islam the state religion.
Debates and open questions
Many senior Shia clerics rejected direct clerical rule, then and now. How religious the revolution was, versus nationalist and economic, is debated.