With the death of Muhammad, the Muslim community was faced with the problem of succession. Who would be its leader? There were four persons obviously marked for leadership: Abu Bakr al-Siddiq, who had not only accompanied Muhammad to Medina ten years before, but had been appointed to take the place of the Prophet as leader of public prayer during Muhammad's last illness; 'Umar ibn al-Khattab, an able and trusted Companion of the Prophet; 'Uthman ibn 'Affan, a respected early convert; and 'Ali ibn Abi Talib, Muhammad's cousin and son-in-law. To avoid contention among various groups, 'Umar suddenly grasped Abu Bakr's hand, the traditional sign of recognition of a new leader. Soon everyone concurred and before dusk Abu Bakr had been recognized as the khalifah of Muhammad. Khalifah- anglicized as caliph - is a word meaning "successor" but also suggesting what his historical role would be: to govern according to the Quran and the practice of the Prophet.

Abu Bakr's caliphate was short but important. An exemplary leader, he lived simply, assiduously fulfilled his religious obligations, and was accessible and sympathetic to his people. But he also stood firm when, in the wake of the Prophet's death, some tribes renounced Islam; in what was a major accomplishment, Abu Bakr swiftly disciplined them. Later, he consolidated the support of the tribes within the Arabian Peninsula and subsequently funnelled their energies against the powerful empires of the East: the Sassanians in Persia and the Byzantines in Syria, Palestine, and Egypt. In short, he demonstrated the viability of the Muslim state.

The second caliph, 'Umar- appointed by Abu Bakr in a written testament - continued to demonstrate that viability. Adopting the title Amir al-Muminin, "Commander of the Believers," 'Umar extended Islam's temporal rule over Syria, Egypt, Iraq, and Persia in what from a purely military standpoint were astonishing victories. Within four years after the death of the Prophet the Muslim state had extended its sway over all of Syria and had, at a famous battle fought during a sandstorm near the River Yarmuk, blunted the power of the Byzantines - whose ruler Heraclius had shortly before disdainfully rejected the letter from the unknown Prophet of Arabia.

Even more astonishingly, the Muslim state administered the conquered territories with a tolerance almost unheard of in that age. At Damascus, for example, the Muslim leader Khalid ibn al-Walid signed a treaty which read as follows:

This is what Khalid ibn al-Walid would grant to the inhabitants of Damascus if he enters therein: he promises to give them security for their lives, property and churches. Their city wall shall not be demolished, neither shall any Muslim be quartered in their houses. Thereunto we give them the pact of Allah and the protection of His Prophet, the caliphs and the believers. So long as they pay the poll tax, nothing but good shall befall them.

This tolerance was typical of Islam. A year after Yarmuk, 'Umar, in the military camp of al-Jabiyah on the Golan Heights, received word that the Byzantines were ready to surrender Jerusalem and rode there to accept the surrender in person. According to one account, he entered the city alone and clad in a simple cloak, astounding a populace accustomed to the sumptuous garb and court ceremonials of the Byzantines and Persians. He astounded them still further when he set their fears at rest by negotiating a generous treaty in which he told them:

In the name of God ... you have complete security for your churches which shall not be occupied by the Muslims or destroyed.

This policy was to prove successful everywhere. In Syria, for example, many Christians who had been involved in bitter theological disputes with Byzantine authorities- and persecuted for it- welcomed the coming of Islam as an end to tyranny. And in Egypt, which 'Amr ibn al-'As took from the Byzantines after a daring march across the Sinai Peninsula, the Coptic Christians not only welcomed the Arabs, but enthusiastically assisted them.

This pattern was repeated throughout the Byzantine Empire. Conflict among Greek Orthodox, Syrian Monophysites, Copts, and Nestorian Christians contributed to the failure of the Byzantines - always regarded as intruders - to develop popular support, while the tolerance which Muslims showed toward Christians and Jews removed the primary cause for opposing them.

'Umar adopted this attitude in administrative matters as well. Although he assigned Muslim governors to the new provinces, existing Byzantine and Persian administrations were retained wherever possible. For fifty years, in fact, Greek remained the chancery language of Syria, Egypt, and Palestine, while Pahlavi, the chancery language of the Sassanians, continued to be used in Mesopotamia and Persia.

'Umar, who served as caliph for ten years, ended his rule with a significant victory over the Persian Empire. The struggle with the Sassanid realm had opened in 687 at al-Qadisiyah, near Ctesiphon in Iraq, where Muslim cavalry had successfully coped with elephants used by the Persians as a kind of primitive tank. Now with the Battle of Nihavand, called the "Conquest of Conquests," 'Umar sealed the fate of Persia; henceforth it was to be one of the most important provinces in the Muslim Empire.

His caliphate was a high point in early Islamic history. He was noted for his justice, social ideals, administration, and statesmanship. His innovations left all enduring imprint on social welfare, taxation, and the financial and administrative fabric of the growing empire.

After the death of 'Umar an advisory council composed of Companions of the Prophet selected as the third caliph 'Uthman, during whose rule the first serious strains on Islamic unity would appear. 'Uthman achieved much during his reign. He pushed forward with the pacification of Persia, continued to defend the Muslim state against the Byzantines, added what is now Libya to the empire, and subjugated most of Armenia. 'Uthman also, through his cousin Mu'awiyah ibn Abi Sufyan, the governor of Syria, established an Arab navy which fought a series of important engagements with the Byzantines.

Of much greater importance to Islam, however, was 'Uthman's compilation of the text of the Quran as revealed to the Prophet. Realizing that the original message from God might be inadvertently distorted by textual variants, he appointed a committee to collect the canonical verses and destroy the variant recensions. The result was the text that is accepted to this day throughout the Muslim world.

Photo: This eighth century manuscript from Mecca or Medina is one of the two oldest known existing copies of the Quran.

These successes, however, were qualified by serious administrative weaknesses. 'Uthman was accused of favoritism to members of his family - the clan of Umayyah. Negotiations over such grievances were opened by representatives from Egypt but soon collapsed and 'Uthman was killed - an act that caused a rift in the community of Islam that has never entirely been closed.

This rift widened almost as soon as 'Ali, cousin and son-in-law of the Prophet, was chosen to be the fourth caliph. At issue, essentially, was the legitimacy of 'Ali's caliphate. 'Uthman's relatives - in particular Mu'awiyah, the powerful governor of Syria, where 'Ali's election had not been recognized - believed 'Ali's caliphate was invalid because his election had been supported by those responsible for 'Uthman's unavenged death. The conflict came to a climax in 657 at Siffin, near the Euphrates, and eventually resulted in a major division between the Sunnis or Sunnites and the Shi'is (also called Shi'ites or Shi'ah), the "Partisans" of 'Ali- a division that was to color the subsequent history of Islam.

Actually the Sunnis and the Shi'is are agreed upon almost all the essentials of Islam. Both believe in the Quran and the Prophet, both follow the same principles of religion and both observe the same rituals. However, there is one prominent difference, which is essentially political rather than religious, and concerns the choice of the caliph or successor of Muhammad.

The majority of Muslims support the elective principle which led to the choice of Abu Bakr as the first caliph. This group is known as ahl alsunnah wa-l-jama'ah, "the people of custom and community," or Sunnis, who consider the caliph to be Muhammad's successor only in his capacity as ruler of the community. The main body of the Shi'is, on the other hand, believes that the caliphate - which they call the imamate or "leadership" - is nonelective. The caliphate, they say, must remain within the family of the Prophet - with 'Ali the first valid caliph. And while Sunnis consider the caliph a guardian of the shari'ah, the religious law, the Shi'is see the imam as a trustee inheriting and interpreting the Prophet's spiritual knowledge.

After the battle of Siffin, 'Ali - whose chief strength was in Iraq, with his capital at Kufa - began to lose the support of many of his more uncompromising followers and in 661 he was murdered by a former supporter. His son Hasan was proclaimed caliph at Kufa but soon afterward deferred to Muiawiyah, who had already been proclaimed caliph in Jerusalem in the previous year and who now was recognized and accepted as caliph in all the Muslim territories - thus inaugurating the Umayyad dynasty which would rule for the next ninety years.

The division between the Sunnis and the Shi'is continued to develop in 680 when Ali's son Husayn along with his followers was brutally killed at Karbala in Iraq by the forces of the Umayyad ruler Yazid. His death is still commemorated every year during the Islamic month of Muharram.