Umar

Umar ibn al-Khattab became the second caliph after Abu Bakr's death. His ten-year reign saw the rapid expansion of the Islamic empire, including the conquest of the Sassanid Empire and Byzantine territories in the Levant and Egypt.

Umar's forces, led by Sa'd ibn Abi Waqqas, defeated the Sassanid army at the Battle of al-Qadisiyyah in 636. This victory led to the Muslim conquest of Iraq and later the fall of the Sassanid capital Ctesiphon, ending Persian imperial rule.

Umar's generals, including Khalid ibn al-Walid and Amr ibn al-As, conquered Byzantine Syria after the Battle of Yarmouk (636) and Egypt after the surrender of Babylon Fortress (641). These conquests added vast territories to the caliphate.

Umar instituted the Hijri calendar, dating from the migration (Hijra) of Muhammad to Medina in 622. This calendar became the standard for the Islamic world, used for religious and administrative purposes.

Umar was stabbed by a Persian slave named Abu Lu'lu'a while leading prayers in Medina. He died three days later, having appointed a council to elect his successor. His death marked the end of the first phase of rapid expansion.