It was in another time — more than 1,300 years ago — in a land known as the Islamic State that, after the death of the Prophet Muhammad, a succession crisis divided Muslims; and the widening schism continues to play out today as ISIL carves its bloody notion of a new Islamic State on the same soil these feuds were first fought, lashing out at targets both within the Muslim world and in the West.

Succession can be a tough adjustment for any group, but is especially emotional when a departing leader of a nascent religion is particularly strong, effective or loved.

A dispute over how to replace Muhammad as the leader of the Muslim world after his death in 632 — and increasingly after the deaths of subsequent leaders — led to competing iterations of the Islamic faith, diverting followers into two major branches — the Sunni and the Shia.

While doctrinal distinctions created the schism, evolving geopolitical notions make it an important matter for world attention.

The split began in the early history of Islam.

Those pushing for selecting successors as caliph of the Islamic State and as the religious authority only from among the family of Muhammad became known as the Shia, from the Arabic for “the followers of Ali,” a reference to Muhammad’s son-in-law, Ali ibn Abi Talib. Those pushing for a selective process based on seeking the most qualified from the wider tribal context became known as the Sunni, from the Arabic for “people of the tradition.”

“This was really a political dispute, but that political dispute early on over who should lead turned into — over centuries — the beginnings of a theological sectarian split,” said Anver Emon, professor of law and Canada Research Chair in religion, pluralism and the rule of law at the University of Toronto

“It is a secession crisis between the two groups, in political terms. But following that, of course, it has had a lot of legacy in terms of religious and legal and ritualistic kinds of distinctions,” said Khalid Mustafa Medani, an associate professor of political science and Islamic studies at Montreal’s McGill University.

What to outsiders may seem an arcane distinction, within Islam can mean everything.

Members of the two branches have lived together peacefully and intermarried, but for some, especially the highly politicized, the divide becomes “very heated” and can even lead to calls for excommunication from the Islamic faith, said Medani.

In the current context of the self-declared Islamic State, that can mean death.

A study in 2009 by the Pew Research Center says there were more than 1.57 billion Muslims around the world, about 23% of the world’s population. Of those, 10 to 13 percent were Shia and 87-90 percent were Sunni.

It is largely where those Shia live that has become important. The majority of Shias (between 68 to 80 percent) live in just four countries: Iran, Pakistan, India and Iraq. In many other countries in the Persian Gulf, Shia remain a minority within Sunni dominated states.

That makes theology increasingly political.

The schism first fully registered in the West in 1979 with the Iranian revolution.

The shah, a secular monarch of Iran, was ousted and replaced by an Islamic republic. The revolutionaries seized the American embassy and held its staff hostage.

That movement was Shia.

The Iranians funded and trained Hezbollah in the 1980s that embarked on a deadly campaign against Israel with suicide bombings, kidnappings and assassinations.

The vast majority of followers of both branches lead peaceful lives — and neither has a monopoly on militancy or moderation. The Iranian revolution and Hezbollah, however, set a popular notion in the West of Shia being the dangerous iteration.

It was a view embraced by many Sunni elites.

“It served the Gulf countries, like Bahrain, like Saudi Arabia, like Kuwait, to strongly associate Shiaism with revolution and thereby raise concerns about their own domestic Shia population,” said Emon. Casting Shiites as heretics aided that domestic need.

Some Sunni leaders also exported their own, competing, brand of the narrative.

It makes the Shia-Sunni split terribly important when the major backers of each branch are dominating influences in the same, sensitive region: Iran and Saudi Arabia.

“To understand the geopolitical rivalries of a country like Iran versus Saudi Arabia is an important way to understand why people are so mobilized,” said Medani, “It is very important to really centre this increasing animosity between Shia and Sunni based on the role of these leaders and the states.”

The Western notion of Shia being the dangerous iteration shifted through fundamentalist Sunni groups such as the Taliban, al Qaeda and the Islamic State of Iraq and the Levant (ISIL, also referred to as ISIS).

For a while, exploiting the Sunni-Shia split served the interests of nations controlled by either branch.

But instability rarely helps heal divides.

In Iraq, for instance, a Sunni minority under Saddam Hussein ruled over a Shia majority. The overthrow of Hussein left people looking to reclaim a lost position and neighbours anxiously eyeing the change.

“For a country like Saudi Arabia, through its exportation of its own Islamist ideology,” said Emon, “they’re responsible for the underlying ideology that informs al Qaeda and ISIS.”

But then ISIL changed the rules.

In 2014, after taking control of territory in the Sunni heartland of Iraq, ISIL proclaimed itself a caliphate, calling other states illegitimate and placing itself as the exclusive authority over the Islamic world, as if the world was the same as it was 1,300 years ago.

ISIL targets Shia Muslims as well as the West as it imposes its strict interpretations within territory it controls.

“ISIS is simply coming home to roost,” said Emon.

The beneficiary of viewing ISIL’s brutal campaign as part of the ancient legacy of Islam, however, is ISIL itself.

ISIL wants to convince everyone the struggle is one epic clash of civilizations between the West and the Islamic World — with themselves as the representative of the world’s Muslims and as their religious authority — their caliph, the scholars said.

“That is the greatest lie that can lead to the greatest conflict,” said Medani.