Which major task completed by first …

After Muhammad’s death, many tribes broke away from the Islamic state. Some even raised their own prophets to establish communities modeled on the umma. Abu Bakr, the first caliph, suppressed the revolts by a series of campaigns. Umar, the second caliph, shaped the umma’s policy of expansion of power. It was not possible to maintain the umma out of the modest income from trade and taxes. So, the caliph and his military commanders mustered their tribal strength to conquer the lands belonging to the Byzantine Empire in the west and the Sasanian Empire in the east.

On the eve of the Arab invasions, these two empires had declined in strength due to religious conflicts and revolts by the aristocracy. In three successful campaigns (637-42), the Arabs brought Syria, Iraq, Iran and Egypt under the control of Medina. The success of the Arabs was contributed by military strategy, religious fervor and the weakness of the opposition. The third caliph, Uthman, launched further campaigns and extended the control to Central Asia. Within a decade of the death of Muhammad, the Arab-Islamic state controlled the vast territory between the Nile and the Oxus.