Western politicians rarely acknowledge the schism between Shia and Sunni Islam. There is nothing remotely comparable to this schism in any other religion in the modern world.

The Sunni-Shia conflict defines the political structure of the Middle East, from the international rivalry between Iran and Saudi Arabia to the internal politics of Muslim nations. And yet, Western politicians, eager to portray Islam as a “religion of peace,” speak of Muslims as homogenous.

At the hard core of political correctness, Islam is treated more like a race than a religion, a monolithic ethnic bloc like “Hispanics” or “Asians.” Both of those groups are, in turn, diverse populations absurdly squeezed into monoliths for the convenience of left-wing political strategists.

In truth, there are Shiite Muslims who do not think Sunnis count as Muslim at all, and vice versa. Adherents of the more extreme sects within the Sunni and Shia schools view moderate followers of the same basic tradition as apostates.

The Sunni-Shiite Divide

Few Western politicians know the first thing about the Sunni-Shiite rift, which flows from a doctrinal dispute that might seem trivial to modern outsiders. When Mohammed died in the 7th Century, there was a profound disagreement among the early followers of Islam about who should succeed him as leader.

The heart of the Sunni-Shiite conflict is that the Sunnis thought the new leader or “caliph” should be elected and chose Mohammed’s close friend Abu Bakr. The leader of the Islamic State, who styles himself as “caliph” or ruler of all true Muslims, calls himself “Abu Bakr al-Baghdadi” in homage to the first caliph. His real name is Ibrahim Awwad Ibrahim al-Badri.

The dissident group we now know as Shiites insisted that only a blood relative of Mohammed was fit to lead, rallying behind Ali bin Abu Talib, who was both Mohammed’s cousin and son-in-law. Ali actually took a turn as caliph after Abu Bakr died, so it would be more precise to say the enduring rift within Islam was caused by Ali’s assumption of leadership and the argument over his successor.

A great deal of 7th-century tribal politics swirled around this conflict, making it more complex than any brief summary could capture. Among other factors, there was Islam’s development into a warrior religion, leading to clan rivalries and vicious arguments over plunder. Personal loyalties to Ali or his rivals played a role as well.

But this is a religious schism, not a matter of stimulating debate between historians. Shiites believe stealing leadership away from the lineal descendants of Mohammed was apostasy, a sin against the true faith.

Ali was assassinated, stabbed in the forehead with a poison sword while praying. Modern Shiites still make a pilgrimage to the mosque where they believe he died and is entombed, located in what is now Iraq. The city where it is located, Najaf, has been the scene of much sectarian bloodshed. The Sunni government of Iraqi dictator Saddam Hussein enraged a generation of Shiites by abusing the Imam Ali mosque.

Ali did not win the title of “caliph” in an election, either. Abu Bakr only reigned for a few years before he died. Ali got the job after Abu Bakr’s second successor, Caliph Uthman, was killed by his own troops in the Muslim holy city of Medina. One reason the Sunni-Shiite divide is so bitter is that Sunnis of the time were furious at Ali for accepting the title of caliph instead of punishing Uthman’s killers.

Followers of Uthman thought Ali committed acts of blasphemy and arrogance against true Islam, and Ali’s followers felt the same way about the Sunni elite. A major point of contention was, and remains, whether Ali swore and broke a binding oath of loyalty to the Sunni hierarchy and the caliphs that came before him.

This is not a minor dispute over the life and times of a long-dead historical personage, but a profound question of religious legitimacy.

Iran still believes its theocracy has rightful authority over Islam under the Shiite model of descent from Mohammed, for example. One of the candidates in the recent Iranian presidential election, cleric Ebrahim Raisi, wears a black turban to signify he is a sayed, a descendant of Mohammed. Raisi choose green as his campaign color because he wanted to take the color back from the secular “Green Movement” demonstrators and restore its “real meaning” as the color of “the revolutionary grandsons of the Prophet.” Those grandsons attempted a revolution against the early Sunni caliphs. They did not die of old age.

Sunni and Shia share many essential beliefs, but even their shared beliefs can be sources of tension. Both Sunnis and Shiites make pilgrimages to the holy cities in Saudi Arabia. Iran frequently castigates the Sunni Saudis over their management of the hajj pilgrimage, alleging discrimination against Shiites along with poor event management. The Saudis supply plenty of poor event management to complain about.

The royal family of Jordan is seen by some analysts as key to bridging the Sunni-Shiite divide, because the Hashemite ruling dynasty of Sunni Jordan claims direct descent from Mohammed’s family, satisfying the Shiite criteria for authentic leadership of Islam. Unfortunately, this also means the Jordanian regime gets to enjoy the violent hatred of both Sunni and Shiite extremists. The Sunni Islamic State infamously burned a captured Jordanian pilot alive in a cage and spread the image across the Internet as one of its favorite propaganda videos. Jordanian officials have nevertheless said they regard the Islamic Republic of Iran as a greater threat to their security than ISIS or other Sunni extremists.

Non-Sunni Minorities

Syria’s dictator, Bashar Assad, is a member of the small Alawite subsect of Shia Islam. Alawites only make up about ten percent of Syria’s population, but the Assad regime, under both Bashar and his father Hafez, consolidated power by appointing Alawites to high government positions. The vast majority of the Syrian population is not Alawite, or even Shiite, but Sunni. Bashar Assad frequently responds to criticism of his brutality by pointing to his history of protecting Syrian religious minorities, including Christians, and noting he belongs to a minority himself.

What is the difference between an Alawite and a Shiite? There are many minor differences in custom and tradition, but the major difference concerns Imam Ali. Recall that Shiites revere Ali as the rightful leader of Islam who should have succeeded Mohammed, and was divinely martyred in death, while Sunnis regard him as a traitor. The Alawites believe he was God incarnate. Some Sunni religious leaders consider them “worse infidels than Christians and Jews,” as one prominent cleric of the Sunni Muslim Brotherhood put it in 2013 when calling for a Sunni jihad against them.

Another branch of Islam that often suffers discrimination and violence from other Muslims is the Sufi sect. The Sufis are neither Sunni nor Shiite — or they might say they are both, since both Sunni and Shiite Islam have Sufi chapters. This makes them an abused minority in both Shiite nations like Iran and Sunni countries like Egypt.

Sufism is more defined by its approach than specific doctrines, unlike the way Sunni and Shia or Shia and Alawite are distinguished. Modern Sufi have a reputation for gentleness and moderation, although they were a formidable military force in the past. The famed “whirling dervish” swordsmen of antiquity were a Sufi invention. Dervishes still whirl, but now the practice is seen as performance art or a form of moving meditation, like tai chi.

Sufi are generally less interested in strict interpretations of the Koran and Islamic sharia law, which makes them despised by hardcore Islamist sects. They are sometimes accused of diluting pure Islam with mystical mumbo-jumbo, or serving as agents for Western powers, seeking to subvert and “tame” true Islam as part of a Western imperialist agenda.

None of these branches of Islam are themselves homogeneous. There are dozens of different Sufi orders, for instance. Some of them are militant or political in nature, contrary to the general impression of Sufis as peaceable mystics.

Sunni Minorities

A school of Sunni Islam that has become increasingly important to American and European politics is Hizmet, a highly organized group founded and led by an imam named Fethullah Gulen. The government of Turkey sees Hizmet as far too organized, prosecuting it (literally) as a vast criminal conspiracy that attempted to overthrow President Recep Tayyip Erdogan last year. The Turkish government refers to Hizmet as “FETO,” an acronym for “Fethullah Terrorist Organization.” Turkey’s diplomatic relations with both Europe and the United States have been rocked by its pursuit of Hizmet and Gulen, who lives in Pennsylvania.

Sunni Islam also includes a movement known as the Salafi, the Islamic fundamentalists. Salafists believe Mohammed, and to a lesser extent his first two generations of descendants, were perfect human beings who should be emulated in every way, including dress and personal hygiene. Salafism includes its own, even more primitive and regressive sub-sects, including the Wahabbi Islam promoted by Saudi Arabia and the Islamic State’s apocalyptic belief system.

“Primitive” is not a pejorative term – Wahabbi Muslims literally embrace the primitive lifestyle of the 7th Century, when Mohammed lived. Their hostility to modernity is one of their defining attributes. Another is their hostility to all other variations of Islam, most definitely including Shiites.

The rapid spread of Salafist beliefs through well-financed overt and covert networks — Salafist madrassas, and agents of influence sent to infiltrate more moderate Islamic schools — is one of the major security concerns of our age, for those analysts and officers who have not been intimidated out of discussing it.

Islam and the West

That brings us back to the problem of sterilizing Islam by treating it as homogenous. The Sunni Muslim Brotherhood has been considered for designation as a terrorist organization by the U.S. government, but its defenders say not even the Brotherhood is a single entity. They insist it has many chapters, many of which cannot be fairly regarded as extremists or terrorists.

To be sure, not all Muslims feel any of this doctrinal animosity. It would be a fool’s game to say “most do” or “most don’t,” given the size of the global Muslim population, the differences between Muslims of different nationalities and ethnic backgrounds, and the effects of emigration and assimilation.

In his speech in Saudi Arabia, President Trump observed that Muslims are often the victims of Islamic terrorism:

In sheer numbers, the deadliest toll has been exacted on the innocent people of Arab, Muslim and Middle Eastern nations. They have borne the brunt of the killings and the worst of the destruction in this wave of fanatical violence. Some estimates hold that more than 95 percent of the victims of terrorism are themselves Muslim.

This is true, but also an incomplete picture of the problem. Muslims abuse and kill each other over doctrinal conflicts on a horrifying scale. Most of that violence and oppression is not “terrorism.” It comes from military conflicts and government crackdowns on religious minorities.

Sectarian strife is one of the reasons why so many Syrian rebel groups viewed favorably by the West are willing to ally with al-Qaeda and other terrorist organizations. In Iraq, there are Sunnis living in territory captured by ISIS that openly welcomed their ghastly conquerors, or were at least reluctant to work with the Iraqi government, because they distrusted the now-Shiite-dominated Iraqi government, and were terrified of the Iran-supported Shiite militias operating in the region.

In Bahrain, the government is under fire for suppressing the Shiite majority in its population, with five dead in a recent police raid against a Shiite community. The Bahraini monarchy, in turn, credibly accuses Iran of seeking to destabilize the country by exacerbating Sunni-Shiite tensions. Bahrain’s Sunnis fear they would be brutalized on an epic scale if Shiites overthrow the government.

This all becomes America’s problem because our national interests in the Middle East are tangled inexorably with the Sunni-Shiite schism. Bahrain, for example, is the strategically vital home to the U.S. 5th Fleet. Shiites resent America for supporting the Sunni monarchy. American military planners are understandably nervous about the prospect of renting a base for the 5th Fleet from a post-revolutionary Bahrain that would be a Shiite satellite of Iran, to say nothing of the cascade effect such a religious war would have on other Sunni allies in the region.