The Muslim conquest of Persia, also known as the Arab conquest of Iran, was carried out by the Rashidun Caliphate from 633 to 654 AD and led to the fall of the Sasanian Empire as well as the eventual decline of the Zoroastrian religion. The rise of the Muslims in Arabia coincided with an unprecedented political, social, economic, and military weakness in Persia. Once a major world power, the Sasanian Empire had exhausted its human and material resources after decades of warfare against the Byzantine Empire. The Sasanian state's internal political situation quickly deteriorated after the execution of King Khosrow II in 628. Subsequently, ten new claimants were enthroned within the next four years. Following the Sasanian civil war of 628–632, the empire was no longer centralized. Arab Muslims first attacked Sasanian territory in 633, when Khalid ibn al-Walid invaded Mesopotamia (then known as the Sasanian province of Asōristān; roughly corresponding to modern-day Iraq), which was the political and economic centre of the Sasanian state. Following the transfer of Khalid to the Byzantine front in the Levant, the Muslims eventually lost their holdings to Sasanian counterattacks. The second Muslim invasion began in 636, under Sa'd ibn Abi Waqqas, when a key victory at the Battle of al-Qadisiyyah led to the permanent end of Sasanian control west of modern-day Iran. For the next six years, the Zagros Mountains, a natural barrier, marked the border between the Rashidun Caliphate and the Sasanian Empire. In 642, Umar ibn al-Khattab, then-Caliph of the Muslims, ordered a full-scale invasion of Persia by the Rashidun army, which led to the complete conquest of the Sasanian Empire by 651. Directing from Medina, a few thousand kilometres away, Umar's quick conquest of Persia in a series of well-coordinated, multi-pronged attacks became his greatest triumph, contributing to his reputation as a great military and political strategist. In 644, prior to the complete annexation of Persia by the Arab Muslims, Umar was assassinated by Abu Lu'lu'a Firuz, a Persian craftsman who was captured in battle and brought to Arabia as a slave. Some Iranian historians have defended their forebears using Arab sources to illustrate that "contrary to the claims of some historians, Iranians, in fact, fought long and hard against the invading Arabs." By 651, most of the urban centres in Iranian lands, with the notable exception of the Caspian provinces (Tabaristan and Transoxiana), had come under the domination of