Umar or Omar, c.581-644, 2d caliph (see caliphate). At first hostile to Islam, he was converted by 618, becoming an adviser to Muhammad. He succeeded Abu Bakr as caliph without opposition in 634. In his reign Islam became an imperial power. The Muslim generals pushed conquests far and wide—into Syria, Egypt, and the Persian Empire. Umar also laid the administrative base of the empire, creating the office of kadi and establishing fixed taxes. He reopened the canals of Mesopotamia and the waterway from the Nile to the Red Sea. Umar was assassinated by a foreign slave. He had appointed a group to select his successor, and the choice fell on Uthman.