(c. 581–644 ad)
Second caliph of Islam (634–44). He presided over the first major wave of Arab conquests, which were the work of great captains such as Khalid ibn al-Walid. Hostile at first to Muhammad, he became an ardent convert. The bond between them was strengthened by Muhammad's marriage to Umar's daughter Hafsa. His genius was administrative rather than military and his achievements included systematizing the rule of his vast territories, establishing the Islamic calendar, organizing state pensions, and upholding justice.