Violence between Islam’s sects has been rare historically, with most of the deadly sectarian attacks directed by clerics or political leaders rather than erupting spontaneously. Extremist groups, many of which are fostered by states, are the chief actors in sectarian killings today.

Two of the most prominent terrorist groups, Sunni al-Qaeda and Shia Hezbollah, have not defined their movements in sectarian terms, and have favored using anti-imperialist, anti-Zionist, and anti-American frameworks to define their jihad, or struggle. They share few similarities beyond the use of violence. Hezbollah has developed a political wing that competes in elections and is part of the Lebanese government, a path not chosen by al-Qaeda, which operates a diffuse network largely in the shadows. Both groups have deployed suicide bombers, and their attacks shifted from a focus on the West and Israel to other Muslims, such as al-Qaeda’s killing of Shia civilians in Iraq and Hezbollah’s participation in the Syrian civil war.

Conflict and chaos have played a role in the reversion to basic sectarian identity. In Iraq, for instance, remnants of Hussein's Ba’athist regime, as well as militants whose organization would eventually become the self-proclaimed Islamic State, employed Sunni rhetoric to mount a resistance to the rise of Shia power. Sunni fundamentalists, many inspired by al-Qaeda’s call to fight Americans, flocked to Iraq from Muslim-majority countries, attacking coalition forces and many Shia civilians. Abu Musab al-Zarqawi, who founded al-Qaeda’s franchise in Iraq, evoked ancient anti-Shia fatwas, or religious rulings, to spark a civil war in hopes that the Shia majority would eventually capitulate in the face of Sunni extremist violence. Iraq's foremost Shia religious authority, Grand Ayatollah Ali al-Sistani, has been a voice for sectarian restraint in Iraq, and the country's Shia community absorbed thousands of deaths before fighting back with their own militias. But, during the U.S. occupation of Iraq and, more recently, offensives against the Islamic State, Shia paramilitaries have been accused of possible war crimes.

Syria’s civil war, in which a quarter million people have been killed and eleven million—more than half the country's prewar population—displaced, has amplified sectarian tensions to unprecedented levels. The war began with peaceful protests in 2011 calling for an end to the Assad regime. Decades of the Assad family's repression of Syria's majority Sunni population and elevation of minority Alawis in government and the private sector has sown sectarian strife. The 2011 protests and brutal government crackdown uncovered sectarian tensions, which have rippled across the region.

Tens of thousands of Syrian Sunnis joined rebel groups such as Ahrar al-Sham, the Islamic Front, and al-Qaeda’s Nusra Front, which all employ anti-Shia rhetoric; similar numbers of Syrian Shias and Alawis enlisted with an Iran-backed militia known as the National Defense Force to fight for the Assad regime. Sunni fighters from Arab and Western countries initially joined the Syrian rebels before turning their guns on them in an effort to establish their envisaged caliphate. Meanwhile Hezbollah and some Shia militias from Iraq, such as Asaib Ahl al-Haq and Kata'ib Hezbollah, backed the Syrian government. Syria’s civil war has attracted more militants from more countries than were involved in the conflicts in Afghanistan, Chechnya, and Bosnia combined.

Al-Qaeda in Iraq had been decimated by Sunni Iraqis who joined the fight against extremists, the U.S.-led military surge, and the death of Zarqawi, its leader, in a 2006 U.S. airstrike, but found new purpose exploiting the vacuum left by the receding Syrian state. It established its own transnational movement known as the Islamic State in Iraq and Syria. The group expanded its grip on Sunni provinces in Iraq and eastern regions in Syria, seizing Iraq’s second-largest city, Mosul, in June 2014. It defied orders from al-Qaeda’s top commanders to curtail its transnational ambitions and wanton violence against civilians, which led to the militant group’s expulsion from al-Qaeda in February 2014. ISIS rebranded as the Islamic State in July 2014 and declared its leader, Abu Bakr al-Baghdadi, as caliph. The group's highly publicized killing of Western hostages triggered a campaign of air strikes by the United States and its regional allies Jordan, Qatar, Saudi Arabia, and the United Arab Emirates.

Extremist groups have come to rely on satellite television and high-speed Internet over the past two decades to spread propaganda and attract recruits. Fundamentalist Sunni clerics, many sponsored by wealthy Sunnis from Saudi Arabia and the Gulf states, have popularized anti-Shia slurs. Shia religious scholars have also taken to the airwaves, mocking and cursing the first three caliphs and Aisha, one of Mohammed’s wives.

Sectarian rhetoric dehumanizing the “other” is centuries old. But the volume is increasing. Dismissing Arab Shias as Safawis, a term that paints them as Iranian agents (from the Safavid empire) and hence traitors to the Arab cause, is increasingly common in Sunni rhetoric. Hard-line Sunni Islamists have used harsher historic terms, such as rafidha, rejecters of the faith, and majus, Zoroastrian or crypto Persian, to describe Shias as heretical. Iranian officials, Iraqi politicians, and Hassan Nasrallah, the leader of Hezbollah, routinely describe their Sunni opponents as takfiris (referring to the doctrine embraced by al-Qaeda of declaring fellow Muslims apostate) and Wahhabis (referring to the puritanical Saudi sect). This cycle of demonization has been amplified throughout the Muslim world.

For Sunni extremists, social media has revolutionized recruitment opportunities. Fundamentalists no longer have to infiltrate mainstream mosques to attract recruits surreptitiously, but can now disseminate their call to jihad and wait for potential recruits to contact them. Shia groups can count on state support from the Iranian, Iraqi, and Syrian governments to recruit militants for sectarian jihad.