Shia identity is rooted in victimhood over the killing of Husayn, the Prophet Mohammed’s grandson, in the seventh century, and a long history of marginalization by the Sunni majority. Islam’s dominant sect, which roughly 85 percent of the world’s 1.6 billion Muslims follow, viewed Shia Islam with suspicion, and extremist Sunnis have portrayed Shias as heretics and apostates.

Sunni and Shia Muslims have lived peacefully together for centuries. In many countries it has become common for members of the two sects to intermarry and pray at the same mosques. They share faith in the Quran and the Prophet Mohammed’s sayings and perform similar prayers, although they differ in rituals and interpretation of Islamic law.

Alongside the proxy battle is the renewed fervor of armed militants, motivated by the goals of cleansing the faith or preparing the way for the return of the messiah. Today there are tens of thousands of organized sectarian militants throughout the region capable of triggering a broader conflict. And despite the efforts of many Sunni and Shia clerics to reduce tensions through dialogue and counterviolence measures, many experts express concern that Islam’s divide will lead to escalating violence and a growing threat to international peace and security.

Islam’s schism, simmering for fourteen centuries, doesn’t explain all the political, economic, and geostrategic factors involved in these conflicts, but it has become one prism through which to understand the underlying tensions. Two countries that compete for the leadership of Islam, Sunni Saudi Arabia and Shia Iran, have used the sectarian divide to further their ambitions. How their rivalry is settled will likely shape the political balance between Sunnis and Shias and the future of the region, especially in Syria, Iraq, Lebanon, Bahrain, and Yemen.

An ancient religious divide is helping fuel a resurgence of conflicts in the Middle East and Muslim countries. Struggles between Sunni and Shia forces have fed a Syrian civil war that threatens to transform the map of the Middle East, spurred violence that is fracturing Iraq, and widened fissures in a number of tense Gulf countries. Growing sectarian clashes have also sparked a revival of transnational jihadi networks that poses a threat beyond the region.

Source: Pew Research, The Future of the Global Muslim Population, 2011

Mohammed unveiled a new faith to the people of Mecca in 610. Known as Islam, or submission to God, the monotheistic religion incorporated some Jewish and Christian traditions and expanded with a set of laws that governed most aspects of life, including political authority. By the time of his death in 632, Mohammed had consolidated power in Arabia. His followers subsequently built an empire that would stretch from Central Asia to Spain less than a century after his death. But a debate over succession split the community, with some arguing that leadership should be awarded to qualified individuals and others insisting that the only legitimate ruler must come through Mohammed’s bloodline.

A group of prominent early followers of Islam elected Abu Bakr, a companion of Mohammed, to be the first caliph, or leader of the Islamic community, over the objections of those who favored Ali ibn Abi Talib, Mohammed’s cousin and son-in-law. The opposing camps in the succession debate eventually evolved into Islam’s two main sects. Shias, a term that stems from shi’atu Ali, Arabic for “partisans of Ali,” believe that Ali and his descendants are part of a divine order. Sunnis, meaning followers of the sunna, or “way” in Arabic, of Mohammed, are opposed to political succession based on Mohammed’s bloodline.

Ali became caliph in 656 and ruled only five years before he was assassinated. The caliphate, which was based in the Arabian Peninsula, passed to the Umayyad dynasty in Damascus and later the Abbasids in Baghdad. Shias rejected the authority of these rulers. In 680, soldiers of the second Umayyad caliph killed Ali’s son, Husayn, and many of his companions in Karbala, located in modern-day Iraq. Karbala became a defining moral story for Shias, and Sunni caliphs worried that the Shia Imams—the descendants of Husayn who were seen as the legitimate leaders of Muslims (Sunnis use the term “imam” for the men who lead prayers in mosques)—would use this massacre to capture public imagination and topple monarchs. This fear resulted in the further persecution and marginalization of Shias.

Even as Sunnis triumphed politically in the Muslim world, Shias continued to look to the Imams—the blood descendants of Ali and Husayn—as their legitimate political and religious leaders. Even within the Shia community, however, there arose differences over the proper line of succession. Mainstream Shias believe there were twelve Imams. Zaydi Shias, found mostly in Yemen, broke off from the majority Shia community at the fifth Imam, and sustained imamate rule in parts of Yemen up to the 1960s. Ismaili Shias, centered in South Asia but with important diaspora communities throughout the world, broke off at the seventh Imam. Most Ismailis revere the Aga Khan as the living representative of their Imam. The majority of Shias, particularly those in Iran and the eastern Arab world, believe that the twelfth Imam entered a state of occultation, or hiddenness, in 939 and that he will return at the end of time. Since then, “Twelvers,” or Ithna Ashari Shias, have vested religious authority in their senior clerical leaders, called ayatollahs (Arabic for “sign of God”).

Many Christian, Jewish, and Zoroastrian converts to Islam chose to become Shia rather than Sunni in the early centuries of the religion as a protest against the ethnic Arab empires that treated non-Arabs as second-class citizens. Their religions influenced the evolution of Shia Islam as distinct from Sunni Islam in rituals and beliefs.

Sunnis dominated the first nine centuries of Islamic rule (excluding the Shia Fatimid dynasty) until the Safavid dynasty was established in Persia in 1501. The Safavids made Shia Islam the state religion, and over the following two centuries they fought with the Ottomans, the seat of the Sunni caliphate. As these empires faded, their battles roughly settled the political borders of modern Iran and Turkey by the seventeenth century, and their legacies resulted in the current demographic distribution of Islam’s sects. Shias comprise a majority in Iran, Iraq, Azerbaijan, and Bahrain, and a plurality in Lebanon, while Sunnis make up the majority of more than forty countries from Morocco to Indonesia.

Modern Tensions