Muslim conquest of Persia
Amidst Persia's turmoil, the first Rashidun invasion of Sasanian territory took place in 633, when the Rashidun army conquered parts of Asoristan, which was the Sasanians' political and economic centre in Mesopotamia. Later, the regional Rashidun army commander Khalid ibn al-Walid was transferred to oversee the Muslim conquest of the Levant, and as the Rashidun army became increasingly focused on the Byzantine Empire, the newly conquered Mesopotamian territories were retaken by the Sasanian army. The second Rashidun invasion began in 636, under Sa'd ibn Abi Waqqas, when a key victory at the Battle of al-Qadisiyyah permanently ended all Sasanian control to the west of modern-day Iran. For the next six years, the Zagros Mountains, a natural barrier, marked the political boundary between the Rashidun Caliphate and the Sasanian Empire. In 642, Umar ibn al-Khattab, eight years into his reign as Islam's second caliph, ordered a full-scale invasion of the rest of the Sasanian Empire. Directing the war from the city of Medina in Arabia, Umar's quick conquest of Persia in a series of coordinated and multi-pronged attacks became his greatest triumph, contributing to his reputation as a great military and political strategist. In 644, however, he was assassinated by the Persian craftsman Abu Lu'lu'a Firuz, who had been captured by Rashidun troops and brought to Arabia as a slave.
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