The Islamic World to 1600
Before his death, Abu Bakr named Umar, another of Muhammad's fathers-in-law, as his successor. That appointment, unlike that of the previous caliph, appears to have gone virtually unchallenged. Umar added Amir al-Mu'minin, meaning "Commander of the Faithful," to his title, and from then on, all caliphs used this title. It denoted the fact that caliphs were not just the political leaders of the Muslim community, but the spiritual leaders as well. Despite this new title, however, Umar is remembered more for his military leadership than his spiritual leadership, because he focussed on expanding the realm of Islam outside of the Arabian peninsula. This focus on the secular would later reappear under the Umayyad dynasty.
The first territorial conquests Umar made were in Syria, which he took from the Byzantines in 635. Damascus, an important city in Syria, fell to the Muslim forces that year, and Jerusalem - considered by Muslims, Christians, and Jews alike to be a holy city - followed in 637. The Muslim policy of tolerance towards other religions had a positive effect on the people of Syria, especially the Christians and Jews, who had been persecuted under the Byzantines. Umar realised that the loyalty of his new subjects was paramount to the success of Islamic rule in the region, and he therefore tried not to alienate them with excessive taxation or oppression. He instituted the kharaj, a tax that landowners and peasants paid according to the productivity of their fields, as well as the jizya, paid by non-Muslims in return for the freedom to practice their own religion. He retained the civil service of the Byzantines, however, until he could establish his own system for governing his rapidly expanding empire, and for that reason Greek remained the language of administration in the new Muslim territories for over 50 years after the conquest.
Umar realised the importance of creating a buffer zone around all of Arabia, the birthplace of Islam, and so while Syria was being invaded to the west, Muslim forces were also heading east through Iraq towards Persia, in an attempt to topple the 400-year-old Sassanid Empire there. The Sassanids were weak at the time of the Muslim invasion, having suffered a recent defeat to the Byzantines, and with an eleven-year-old boy on the throne. Despite this advantage, fighting Syrian forces to the west and Persian forces to the east proved difficult. Once Umar had consolidated his power in Syria, however, he was able to devote his full attention to fighting the Sassanids, and in 636 the Muslims won the decisive Battle of Qadisiyya near the Euphrates River. From there, the Muslims moved further east to occupy Ctesiphon, the Sassanid capital, on the Tigris River. Muslim forces continued further east into Persia, conquering one city after another. By 653, nine years after Umar's death, they reached the Oxus River in Central Asia, the eastern border of the Sassanid Empire.
After conquering Ctesiphon about 637, the Muslim forces again turned west, looking to spread the new faith into Egypt. With Syria firmly in Muslim hands, the army had no trouble crossing the Sinai Peninsula into Egypt. In 641 the ancient fortress of Babylon, south of present-day Cairo, fell to the invaders, and in 642 the Byzantine Patriarch Cyrus agreed to the surrender of Alexandria, on the Nile Delta. In 645 the Byzantines briefly reconquered that city, but the following year the Muslims reclaimed it. Christians never again ruled in Egypt.
The kharaj and jizya taxes, which had already been implemented in Syria after its conquest, were also introduced in Persia and Egypt. While Islamic tolerance towards other religions may seem surprising, considering the general practice of Christians to persecute people of other religions living in their empires, it was a policy based more on financial logic than religious indifference. Although from a religious perspective, an all-Muslim population would have been ideal, the new Islamic empire needed the funds provided by the jizya, which was paid by all non-Muslims. Hence, actively attempting to convert large numbers of their new subjects to Islam would have severely decreased the empire's coffers. Additionally, the Muslim conquerors did not want to trigger a revolt when they were so outnumbered by non-Muslims in their new territories. For these reasons, the Islamic rulers did not usually promote proselytising of their religion.
In newly conquered Persia, the Muslims faced different problems from the fiercely nationalistic people there. With a strong language, culture, and, especially, religion of its own, the Persians greatly resented their conquest by the despised Arabs. The Sassanid Empire had its own state religion, Zoroastrianism, and unlike the patchwork of religions that existed in Syria and Egypt under the Byzantines, most Persians adhered to Zoroastrianism as not only their state religion, but also as an integral part of their culture and identity. They considered the Arab Muslims an inferior people with an inferior civilisation. The Arabs and Persians were almost constantly in conflict with each other, even after many Persians converted to Islam, and this conflict created a number of problems for the ruling Arabs as the Islamic world continued to grow outside of Arabia.
The first sign of conflict between the Arabs and Persians came with the assassination of the Caliph Umar by a Persian Christian in 644. During his ten-year rule, Umar had furthered Abu Bakr's territorial gains in Arabia by conquering a great expanse of land surrounding the peninsula: Syria, Egypt, Iraq, and much of Persia. In the process he created the second largest empire in the world at the time, only slightly smaller than the Chinese Empire. Only twelve years after Muhammad's death, Islam had become a major player on the world stage.
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