Umar ibn al-Khattab

Umar ibn al-Khattab ( 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.