The young Muslim community faced a pivotal decision after the prophet’s death early in the seventh century in what is now Saudi Arabia.
The “Meccan elite” wanted to pick a successor according to their tribal customs, with candidates drawn from a handful of elders, said Juan Cole, an expert on modern Middle East history at the University of Michigan. “The people who rose on their list tended to be intermarried with (the prophet’s) family, but not directly related.”
Two hundred miles north in Medina, where Muhammad had settled after he left Mecca, a more dynastic view favored a blood relative of the religion’s founder. The closest male relative was Ali, Muhammad’s first cousin and son-in-law.
The Meccans prevailed, and Abu Bakr, the prophet’s father-in-law, became the first caliph, or head of Islam. Ali became the fourth caliph in 656, but the question of who should rule rose again after his assassination five years later.
A relative of the third caliph became the next leader, and he moved the caliphate to Damascus, Syria.
About this time Muslims who had been followers of Ali objected to what they saw as a shift away from Muhammad’s teachings. The people of Kufa, in what is now Iraq, called on Ali’s second son, Hussein, to lead them.
Hussein and 72 people, including his family, ultimately faced the caliph’s army of thousands in the fields of Karbala. Cole said they were surrounded and killed.
Shias believe Hussein, the prophet’s grandson, sacrificed his life for Islam, a concept scholars refer to as the “Karbala paradigm.” It explains lingering feelings of persecution and martyrdom and a desire to fight perceived injustices.
The sectarian character of the Sunni-Shia split, Cole said, became firm after Hussein’s death in 680. The Sunnis continued to follow the caliphate for several centuries. The Shias looked only to Hussein’s descendants for religious leadership.
Ali, Hussein and their descendants became known as the 12 Imams, who are considered infallible by their followers.
After the 11th imam died in the ninth century, followers believed his young son went into hiding. Since then Shias, also known as the Twelvers, have waited for the 12th imam’s return.
“It’s a little like Christ for the Christians,” Cole said. “They believe he will return and make things better.”
Although the question of Muhammad’s successor is the major religious difference between Sunnis and Shias, other distinctions developed over the centuries.