Arab conquest of Persia and the fall of the Sasanian Empire

The war with Byzantium and the strife that followed the murder of Khosrow II in 628 left the Sasanian state weakened and divided: in four years about ten kings and queens came and went, and real power passed to generals and great noble houses. In 632 the boy Yazdegerd III was placed on the throne. In 633, after the Ridda wars, Caliph Abu Bakr sent Khalid ibn al-Walid into Iraq, where the tribesmen of al-Muthanna ibn Haritha were already raiding; al-Hira surrendered, but when Khalid was sent on to Syria the Persians recovered and in 634 defeated the Arabs at the Battle of the Bridge. In 636 a new army under Sa'd ibn Abi Waqqas met the Persian army of Rustam Farrokhzad at al-Qadisiyyah. After several days of fighting — war elephants on the Persian side, veterans arriving from the Syrian front on the Arab side — Rustam was killed and his army routed; according to tradition, the Arabs captured the royal banner, the Derafsh-e Kaviani.

Ctesiphon fell in March 637 after a siege of two or three months, and after the victory at Jalula the whole of Mesopotamia was in Arab hands; the conquerors settled in the new garrison cities of Basra and Kufa. Yazdegerd withdrew to the Iranian plateau and in 642 gathered a last great army at Nahavand in Media; its defeat opened Iran to the Arabs, who in a series of campaigns directed from Medina by Caliph Umar and continued under Uthman took Isfahan, Fars — Istakhr fell around 650 — and Khorasan. Yazdegerd fled from province to province, vainly seeking allies among the Turks and in Tang China, and in 651 (or 652) he was murdered near Merv by a miller, perhaps at the instigation of the local governor, Mahoe Suri.

The death of Yazdegerd ended more than four centuries of Sasanian rule. Iran became part of the caliphate, although in Tabaristan, south of the Caspian, the local Dabuyid dynasty kept its independence until the Abbasid conquest of 760, and the Islamisation of the country was a gradual process: Islam became its predominant religion only after several centuries, while Zoroastrianism declined. The traditional narrative rests on Arabic histories written long after the events and on the Armenian history attributed to Sebeos, and it has been questioned: the historian Parvaneh Pourshariati sees the Sasanian state as a fragile confederation with the great Parthian houses, several of which made their own peace with the Arabs, and dates the conquest of Mesopotamia to 628–632. Most accounts place al-Qadisiyyah in the autumn of 636 — in late September, mid-November or early December — and some in 637.