The ideal of a unified Muslim community, so important to Muhammad, proved difficult to realize as conquest and conversion vastly enlarged the Islamic umma. A central problem involved leadership and authority in the absence of Muhammad’s towering presence. Who should hold the role of caliph (KAY-lihf), the successor to Muhammad as the political leader of the umma, the protector and defender of the faith? That issue crystallized a variety of emerging conflicts within the Islamic world—between early and later converts, among various Arab tribes and factions, between Arabs and non-Arabs, between privileged and wealthy rulers and their far less fortunate subjects. Many of these political and social conflicts found expression in religious terms as various understandings of the Quran and of Muhammad’s life and teachings took shape within the growing Islamic community.

The first four caliphs, known among most Muslims as the Rightly Guided Caliphs (632–661), were close “companions of the Prophet,” selected by the Muslim elders of Medina. Division surfaced almost immediately as a series of Arab tribal rebellions and new “prophets” persuaded the first caliph, Abu Bakr, to suppress them forcibly. The third and fourth caliphs, Uthman and Ali, were both assassinated, and by 656, less than twenty-five years after Muhammad’s death, civil war pitted Muslim against Muslim.

Out of that conflict emerged one of the deepest and most enduring rifts within the Islamic world. On one side were the Sunni (SOON-nee) Muslims, who held that the caliphs were rightful political and military leaders, selected by the Islamic community. On the other side of this sharp divide was the Shia (SHEE-ah) (an Arabic word meaning “party” or “faction”) branch of Islam. Its adherents felt strongly that leadership in the Islamic world should derive from the line of Ali and his son Husayn, blood relatives of Muhammad, both of whom died at the hands of their political or religious enemies. If the caliph was the idealized communal leader for Sunnis, imams (leaders) served this purpose for most of the Shia Muslims. They were widely thought to have some special charisma based on descent from the Prophet, giving them a religious authority that the caliphs lacked and allowing them to infallibly interpret divine revelation and law.

Thus what began as a purely political conflict acquired over time a deeper significance. For much of early Islamic history, Shia Muslims saw themselves as the minority opposition within Islam. They felt that history had taken a wrong turn and that they were “the defenders of the oppressed, the critics and opponents of privilege and power,” while the Sunnis were the advocates of the established order.11 Various armed revolts by Shias over the centuries, most of which failed, led to a distinctive conception of martyrdom and to the expectation that their defeated leaders were merely in hiding and not really dead and that they would return in the fullness of time. Thus a messianic element entered Shia Islam. The Sunni/Shia schism became a lasting division in the Islamic world, reflected in conflicts among various Islamic states, and was exacerbated by further splits among the Shia. Those divisions echo still in the twenty-first century.

As the Arab Empire grew, its caliphs were transformed from modest Arab chiefs into absolute monarchs, “the shadow of God on earth,” of the Byzantine or Persian variety, complete with elaborate court rituals, a complex bureaucracy, a standing army, and centralized systems of taxation and coinage. They were also subject to the dynastic rivalries and succession disputes common to other empires. The first dynasty, following the era of the Rightly Guided Caliphs, came from the Umayyad (oo-MEYE-ahd) family (r. 661–750). Under its leadership, the Arab Empire expanded greatly, caliphs became hereditary rulers, and the capital moved from Medina to the cosmopolitan Roman/Byzantine city of Damascus in Syria. Its ruling class was an Arab military aristocracy, drawn from various tribes. But Umayyad rule provoked growing criticism and unrest. The Shia viewed the Umayyad caliphs as illegitimate usurpers, and non-Arab Muslims resented their second-class citizenship in the empire. Many Arabs protested the luxurious living and impiety of their rulers. The Umayyads, they charged, “made God’s servants slaves, God’s property something to be taken by turns among the rich, and God’s religion a cause of corruption.”12

Such grievances lay behind the overthrow of the Umayyads in 750 and their replacement by a new Arab dynasty, the Abbasids. With a splendid new capital in Baghdad, the Abbasid caliphs presided over a flourishing and prosperous Islamic civilization in which non-Arabs, especially Persians, now played a prominent role. But the political unity of the Abbasid Empire did not last long. Beginning in the mid-ninth century, many local governors or military commanders effectively asserted the autonomy of their regions, while still giving formal allegiance to the caliph in Baghdad. Long before Mongol conquest put an official end to the Abbasid Empire in 1258, the Islamic world had fractured politically into a series of “sultanates,” many ruled by Persian or Turkish military dynasties.

A further tension within the world of Islam, though one that seldom produced violent conflict, lay in different answers to one central question: what does it mean to be a Muslim, to submit wholly to Allah? That question took on added urgency as the expanding Arab Empire incorporated various peoples and cultures that had been unknown during Muhammad’s lifetime. One answer lay in the development of the sharia, the body of Islamic law developed primarily in the eighth and ninth centuries by religious scholars, Sunni and Shia alike, known as the ulama.

Based on the Quran, the life and teachings of Muhammad, deductive reasoning, and the consensus of scholars, the emerging sharia addressed in great detail practically every aspect of life. It was a blueprint for an authentic Islamic society, providing meticulous guidance for prayer and ritual cleansing; marriage, divorce, and inheritance; business and commercial relationships; the treatment of slaves; political life; personal hygiene; dietary requirements; and much more. Debates among the ulama led to the creation of four schools of law among Sunni Muslims and still others in the lands of Shia Islam. To the ulama and their followers, living as a Muslim meant following the sharia and thus participating in the creation of an Islamic society.

A second and quite different understanding of the faith emerged among those who saw the worldly success of Islamic civilization as a distraction and deviation from the purer spirituality of Muhammad’s time. Known as Sufis (SOO-fees), they represented Islam’s mystical dimension, in that they sought a direct and personal experience of the Divine. Through renunciation of the material world, meditation on the words of the Quran, the chanting of the names of God, the use of music and dance, and the veneration of Muhammad and various “saints,” Sufis pursued an interior life, seeking to tame the ego and achieve spiritual union with Allah. To describe that inexpressible experience, they often resorted to metaphors of drunkenness or the embrace of lovers. “Stain your prayer rug with wine,” urged the famous Sufi poet Hafiz, referring to the intoxication of the believer with the Divine Presence. (See Zooming In: Mullah Nasruddin for an expression of popular or folk Sufism.)

This mystical tendency in Islamic practice, which became widely popular by the ninth and tenth centuries, was at times sharply critical of the more scholarly and legalistic practitioners of the sharia. To Sufis, establishment teachings about the law and correct behavior, while useful for daily living, did little to bring the believer into the presence of God. For some, even the Quran had its limits. Why spend time reading a love letter (the Quran), asked one Sufi master, when one might be in the very presence of the Beloved who wrote it?14 Furthermore, Sufis felt that many of the ulama had been compromised by their association with worldly and corrupt governments. Sufis therefore often charted their own course to God, implicitly challenging the religious authority of the ulama. For these orthodox religious scholars, Sufi ideas and practice sometimes verged on heresy, as Sufis on occasion claimed unity with God, received new revelations, or incorporated novel religious practices from outside the Islamic world.

Despite their differences, adherents of the legalistic emphasis of the sharia and practitioners of Sufi spirituality coexisted, mostly peacefully, mixing and mingling, collaborating and disagreeing, in various combinations. For many centuries, roughly 1100 to 1800, Sufism was central to mainstream Islam, and many, perhaps most, Muslims affiliated with one or another Sufi organization, making use of its spiritual practices. A major Islamic thinker, al-Ghazali (1058–1111), himself both a legal scholar and a Sufi practitioner, in fact worked out an intellectual accommodation among these different strands of Islamic thought. Rational philosophy alone could never enable believers to know Allah, he argued. Nor were revelation and the law sufficient, for Muslims must know God in their hearts, through direct personal encounter with Allah. Nonetheless, differences in emphasis remained an element of tension and sometimes discord within the world of Islam.