Though the two main sects within Islam, Sunni and Shia, agree on most of the fundamental beliefs and practices of Islam, a bitter split between the two goes back some 14 centuries. The divide originated with a dispute over who should succeed the Prophet Muhammad as leader of the Islamic faith he introduced.

Today, about 85 percent of the approximately 1.6 billion Muslims around the world are Sunni, while 15 percent are Shia, according to an estimate by the Council on Foreign Relations. While Shia represent the majority of the population in Iran, Iraq, Bahrain and Azerbaijan and a plurality in Lebanon, Sunnis are the majority in more than 40 other countries, from Morocco to Indonesia.

Despite their differences, Sunni and Shia have lived alongside each other in relative peace for most of history. But starting in the late 20th century, the schism deepened, exploding into violence in many parts of the Middle East as extreme brands of Sunni and Shia Islam battle for both religious and political supremacy.

The roots of the Sunni-Shia divide can be traced all the way back to the seventh century, soon after the death of the prophet Muhammad in A.D. 632. While most of Muhammad’s followers thought that the other elite members of the Islamic community should choose his successor, a smaller group believed only someone from Muhammad’s family—namely his cousin and son-in-law, Ali—should succeed him. This group became known as the followers of Ali; in Arabic the Shiat Ali, or simply Shia.

“The essence of the problem is that Muhammad died without a male heir, and he never clearly stated who he would want to be his successor,” says Lesley Hazleton, author of After the Prophet: The Epic Story of the Sunni-Shia Split in Islam. “This was important, because by the time he died, he had basically brought all the tribes of Arabia together into a kind of confederation that became the ummah—the people or nation of Islam.”

Eventually the Sunni majority (named for sunna, or tradition) won out, and chose Muhammad’s close friend Abu Bakr to become the first caliph, or leader, of the Islamic community. Ali eventually became the fourth caliph (or Imam, as Shiites call their leaders), but only after the two that preceded him had both been assassinated.

Ali, himself, was killed in 661, as the bitter power struggle between Sunni and Shia continued. At stake was not only control of Muhammad’s religious and political legacy, but also a great deal of money, in the form of taxes and tributes paid by the various tribes united under the banner of Islam. This combination of money and power would only grow. Within the century after Muhammad’s death, his followers had built an empire that stretched from Central Asia to Spain.

In 681, Ali’s son Hussein led a group of 72 followers and family members from Mecca to Karbala (present-day Iraq) to confront the corrupt caliph Yazid of the Ummayad dynasty. A massive Sunni army waited for them, and by the end of a 10-day standoff with various smaller struggles, Hussein was killed and decapitated, and his head brought to Damascus as a tribute to the Sunni caliph.

“It was obviously intended by the Ummayads to put the definitive end to all claims to leadership of the ummah as a matter of direct descendence from Muhammad,” says Hazleton of Hussein’s death, and the death of all the surviving members of Muhammad’s family, at Karbala. “But of course it's not what happened.” Instead, Hussein’s martyrdom at Karbala became the central story of Shia tradition, and is commemorated yearly as Ashoura, the most solemn date on the Shia calendar.

In addition to Karbala, the NPR podcast Throughline identified three key milestones that would sharpen Sunni-Shia divisions by the end of the 20th century. First came the rise of the Safavid dynasty in the 16th century, which transformed Iran (through force) from a Sunni center into the Shia stronghold of the Middle East. In the early 20th century, the victorious Allies divided the territory held by the former Ottoman Empire after World War I, cutting through centuries-old religious and ethnic communities in the process. Finally, in 1979, the Islamic Revolution in Iran produced a radical brand of Shia Islam that would clash violently with Sunni conservatives in Saudi Arabia and elsewhere in the decades to follow.

Amid the increasing politicization of Islam and the rise of fundamentalists on both sides of the divide, sectarian tensions intensified in the early 21st century, especially amid the upheavals caused by two Persian Gulf Wars, the chaos that followed the U.S.-backed ouster of Saddam Hussein’s Sunni regime in Iraq, and the mass uprisings across the region that began with the Arab Spring in 2011.

Sunni-Shia divisions would fuel a long-running civil war in Syria, fighting in Lebanon, Iran, Iraq, Yemen and elsewhere, and terrorist violence on both sides. A common thread in most of these conflicts is the ongoing battle between Sunni Saudi Arabia and Shia Iran for influence in the oil-rich Middle East and surrounding regions.

Despite the long-running nature of the Sunni-Shia divide, the fact that the two sects coexisted in relative peace for many centuries suggests their struggles may have less to do with religion than with wealth and power.

“Neither of them are representative of the vast majority of Sunni Muslims or the vast majority of Shia Muslims around the world,” says Hazleton of the fundamentalist regimes governing both Saudi Arabia and Iran.

“When society breaks down, you fall back on old forms of identity, and Shia and Sunni are 1,400-year-old forms of identity.”