The origin of the great schism between the Sunnis and Shi'a is often described as follows: in AD 632, the Prophet Muhammad died without naming a successor. The Sunnis (followers of the "sunna," that is, the Prophet's tradition) believed the Prophet's companions should choose his successor. The Shi'a (those of the "party of Ali") held that Muhammad, directly inspired by God, did choose a successor, specifically, his cousin and son-in-law, Ali ibn Abi Talib. Though Ali served as the fourth leader (caliph) of the Muslims, his leadership and assassination caused an irrevocable division between the Sunnis and Shi'a.

But this is not the entire story. According to Laurence Louër the truth is that the Sunni-Shi'a split is no "mere question over the prophet of Islam's succession." Rather, it centers on the "nature of legitimate political authority" (1). In Louër's telling, the political needs of religious leaders and communities have shaped Islamic dogma since Muhammad's death. Even then, Louër argues, it was the arrival of the Safavid Empire in Iran in 1501, not Ali's death, that caused the decisive split between the Sunni and Shi'a Muslims.

The first half of Sunnis and Shi'a traces the formation of Islam and its several factions. The author illustrates how individuals and religious leaders combined political and religious power for their own and their supporters' benefit. The first caliphs were "religious leaders and exercised a theocratic-type power." They played a central role in "developing religious law, defining ritual, and settling on the text of the Koran" (8). Communities of religious scholars (ulama) who deduced practical rules from the divine law revealed in the Koran, "rapidly became social and political movements" (15). Several states and empires, including the Abbasids, granted the ulama power as a means to legitimize their rule. In the event, the ulama surmounted the caliph's influence and legitimacy.

Spiritual advisers known as imams (those who proceed) became central figures in Shi'ism, thought to reveal the hidden meaning of God's messages to each prophet. In Shi'ism, Ali was the first imam and progenitor of all who followed. According to Louër, the sixth imam, Ja'far al-Sadiq, played a vital role in formalizing the imamate and making imams responsible for administering the everyday affairs of the faithful, as well as developing doctrines and collecting and distributing tax revenues. They became religious scholars and founders of religious schools, thus monopolizing political and social control over their communities.

Today, most Shi'a are "Twelvers," believing the twelfth imam ascended to heaven in AD 941 and will return to "establish the reign of justice and truth" (16). But the breakaway and "protest movements"—like the Ismailis, Fatimids, Qarmatians, Druze, Kharijites—prompted Sunni and Shi'a ulamas to "accelerate the crystallization of Sunni and Shi'i orthodoxies" (21), solidifying doctrines that preached obedience and consolidating myths of the origins of Islam. Because political survival was crucial for these religious leaders, Louër argues, "the Shi'i ulama showed no less political realism than their Sunni counterparts" (29).

When the Safavid Empire (c. 1501–1736) mandated Shi'ism within its borders, the consequent shift from communal to official religion split the Islamic world. Attempts have been made to rectify this rift: nationalist movements like the late nineteenth-century Arab renaissance tried to bridge the gap. Times of crisis, as when Israel was founded, also spurred forlorn hopes for reunification. Saudi Arabia and Egypt's mid-twentieth century "cold war" divided the Arab world. The Iranian Revolution (1979) and the Iraq War (2003) only deepened this sectarian divide.

The second half of the book clarifies how Sunni and Shi'a doctrines and practices have emerged in Iraq, Bahrain, Pakistan, Saudi Arabia, Iran, Yemen, and Lebanon. Louër first looks at Iraq. The Ottoman and Safavid conquests brought the Sunnis and Shi'a into contact and conflict with one another. Karbala (burial site of Hussein) and Najaf (burial site of Ali) are extremely important cities for Shi'a faithful. Their religious significance and growing economic power under the Ottomans strengthened the Shi'a ulamas' power in those cities. When the British took control there, they relied on the Sunnis rather than the Shi'a ulamas, whom they saw as "theocratic despots" (112). Independent Iraq, under King Faysal (and later the Ba'thists), favored Iraqis over ethnic Iranian citizens of Ottoman descent, effectively disenfranchising the Shi'a, who could not hold political office or serve in the military. During the Iranian revolution, the Ba'thists labeled the Shi'a as an Iranian fifth column. Louër argues that this reflected nationalist, not religious, fears (115). When Saddam cracked down on the Shi'a clergy, many fled to the United States. In the lead-up to 2003, these promoted the idea of democracy in Iraq to the American government, while trying to position themselves in the new Iraqi government. The result is that Iraq is now Shi'a dominated.

Bahrain, like Iraq, was conquered and divided by both the Ottomans and Safavids. Settled by Shi'a in the tenth century, the island came to be ruled by the Sunni minority after the Safavid withdrawal. Unlike their counterparts in Iraq, the Shi'a in Bahrain are not considered to be an Iranian fifth column. Instead, Louër maintains, the Bahrain Shi'a have opted for "strategies of avoidance rather than opposition" (126). Al-Khalifa, the ruling Sunni royal family, has indicated their preference for preserving national unity, even if it means irritating its Sunni population.

Outside the Arabian peninsula, Louër writes, Pakistan "is one of the main scenes of anti-Shi'a sectarian violence" (149). Following the partition of Pakistan and India in 1947, Muslim leaders in the new country used Islam as a "function … of secularized national belonging" (139). But fractures soon emerged. Debates swirled over language differences, immigration issues, and such practical matters as how to bury a statesman—according to Sunni or Shia doctrine? From the 1950s to the 1980s, the Pakistani constitution codified Islamic law and enshrined Sunni traditions. Louër stresses the regional context here, as Pakistan's Sunni ascent accelerated as the war in Afghanistan progressed.

Since its revolution, Iran has striven to be the center of Islam while exporting its own ideology. But it privileges a "narrowly defined vision of the Iranian national interest over Islamic solidarity" (86). It has established relationships with Sunni groups (Hamas, Islamic Jihad, and the Palestinian Muslim Brotherhood) and factions Shi'ism considers heretical, including the Alawites of Syria. For Iran, Syria would grant access to Lebanon and provide a united geographic front against Israel (85). In other words, geopolitical strategies outweigh unified religious doctrines.

This pragmatism also materialized in Saudi Arabia. After the American invasion of Iraq, Saudi Arabia watched Iran co-opt Iraqi politics and militias. Louër maintains a fear arose—that the Americans would side with the Shi'a. To avoid a Shi'a "fertile crescent," the Saudis relaxed restrictions on their own Shi'a citizens (98). Al-Saud, the Saudi royal family, then rushed to court the Shi'a elite in Saudi Arabia in order to forestall Iranian influence. Nevertheless, the persecution of several political figures and dissidents continued.

While some have categorized the civil war in Yemen as a proxy war between Saudi Arabia and Iran, "most media outlets [relegate] its extremely complex local issues to the background or simply [ignore] them" (102). Louër clarifies the situation. Forty percent of the population in Yemen adheres to Zaydism, a group that broke from Shi'ism to support the fourth imam's son. Yet, Louër notes, Zaydis occupy the middle between Sunni and Shi'a ideology. For instance, they believe the ruler must be transitioned by consultation (a Sunni belief), but also commemorate Ali as a caliph (a Shi'a practice). Since the 1960s, various Yemeni groups have tried but failed to co-opt the Zaydis (175).

Civil war also affected Islam in Lebanon. Before 1975, tensions in Lebanon were generally between Muslims and Christians. While the French succeeded in co-opting the Sunni population, the Shi'a remained divided over cooperating with the state. However, the Six Day War (1967) changed everything. As thousands of Sunni Palestinians fled to Lebanon, the Shi'a felt increasing pressure to mobilize. The Palestinian Liberation Organization (PLO) worsened the situation by launching attacks against Israel from southern Lebanon. This left Lebanese Shi'a caught between the PLO and Israel. Louër notes that Syria's entry into the civil war on the side of the Christians shattered the Sunni-Shi'a dynamic in Lebanon. Fearing it would not survive a war with Israel, Syria sought to prevent Lebanon from becoming a Sunni-dominated state. The later entry of Saudi Arabia and Iran into Lebanese politics further divided Lebanon.

Louër concludes by arguing that the Sunnis and Shi'a are locked in a "mimetic rivalry." Tensions between the two boil down to "differentiation" and "imitation" (194). Since Muhammad's death, Sunni and Shi'a religious leaders have both emulated and reviled each other.

Sunnis and Shi'a has its shortcomings. One wishes, for instance, that the author had included another state from outside the Arabian Peninsula to strengthen her regional approach. Questions also remain about how Arab culture and identity influenced both the schism and the resulting doctrines. The zealots of the Islamic State of Iraq and Syria, or Daesh, justify their campaigns of rape, pillage, and murder by pointing to the conquests of Muhammad, who himself tied the Arab culture of raiding with political and military power. Since tribal warfare influenced Islam, this warranted further discussion in the first half of the book.

These quibbles aside, Laurence Louër's book, published ten years after the Arab Spring, could not be more timely. Conflict in the Middle East today is too often oversimplified and attributed to clashes between Sunnis and Shi'a. Louër disproves this notion and reveals the complexities of Islam as an ideology and function of the state. Sunnis and Shi'a should be required reading for students and others seeking to better understand the Islamic world.