Every Muslim hoped that by observing the laws of the Qur’an, he or she could achieve salvation, and it was the tenets of Islam preached by Muhammad that bound all Arabs together. Despite the clarity and unifying force of Muslim doctrine, however, divisions developed within the Islamic faith within decades of Muhammad’s death. Neither the Qur’an nor the hadith gave clear guidance about how successors to Muhammad were to be chosen, but, according to tradition, in 632 a group of Muhammad’s closest followers chose Abu Bakr (uh-BOO BAH-kuhr), who was a close friend of the Prophet’s and a member of a clan affiliated with the Prophet’s clan, as caliph (KAY-luhf), a word meaning “successor.” He was succeeded by two other caliphs, but these provoked opposition, which coalesced around Ali, Muhammad’s cousin and son-in-law. Ali was chosen as the fourth caliph in 656, but was assassinated only five years later by backers of the initial line of caliphs. Ali’s supporters began to assert that the Prophet had designated Ali as imam, or leader, and that he should rightly have been the first caliph; thus, any caliph who was not a descendant of Ali was a usurper. These supporters of Ali — termed Shi’ites (SHEE-ights) or Shi’a (SHEE-ah) from Arabic terms meaning “supporters” or “partisans” of Ali — saw Ali and subsequent imams as the divinely inspired leaders of the community. The larger body of Muslims who accepted the first elections — termed Sunnis, a word derived from Sunna, the practices of the community derived from Muhammad’s example — saw the caliphs as political leaders. Since Islam did not have an organized priesthood, the caliphs had an additional function of safeguarding and enforcing the religious law (shari’a) with the advice of scholars (ulama), particularly the jurists, judges, and scholastics who were knowledgeable about the Qur’an and hadith. Over the centuries enmity between Sunni and Shi’a Muslims has sometimes erupted into violence, and discord still exists today.

After the assassination of Ali, the caliphate passed to members of the Umayyad (oo-MIGH-uhd) clan, who asserted control and brought stability to the growing Muslim empire. They established their capital at Damascus in Syria, and the Muslim faith continued to expand eastward to India and westward across North Africa. That expansion was facilitated everywhere by three main factors: military strength, trade connections, and tolerance toward non-Muslims. By the early tenth century a Muslim proverb spoke of the Mediterranean Sea as a Muslim lake, though the Greeks at Constantinople contested that notion.