The Fall of the Sasanian Empire — Rome’s last great rival conquered by the Arab armies

The Sasanian Empire — the last pre-Islamic Persian empire, founded by Ardashir I in 224 and ruled from its winter capital at Ctesiphon on the Tigris — was conquered by the Arab Muslim armies of the Rashidun Caliphate between roughly 633 and 651. Its last shah, Yazdegerd III, a grandson of the great king Khosrow II, fled eastward across his own collapsing realm for nearly a decade and was killed near Merv in 651, by tradition murdered by a local miller for the jewellery he carried. With his death the dynasty that had been Rome’s and then Byzantium’s principal rival for four centuries ceased to exist, and Iran began its long passage from Zoroastrian empire to part of the Islamic world.

The empire the Arabs broke was, like Byzantium that same century, already gravely wounded — and the wound was largely self-inflicted. The Sasanian and East Roman empires had fought a final, ruinous war from 602 to 628 in which Khosrow II overran Syria, Palestine, and Egypt before the emperor Heraclius drove deep into Persia and shattered his armies. Both empires emerged exhausted, bankrupt, and depopulated by war and plague. When Khosrow was murdered by his own nobles in 628, the Sasanian state fell into a four-year civil war in which a dozen or more claimants — including two queens, Boran and Azarmidokht — passed across the throne before the nobility agreed on the boy Yazdegerd III in 632.

It was onto this enfeebled, faction-ridden empire, not a Persia at its height, that the Arab armies advanced. The conquest came as a sequence of decisive battles against a state that could no longer absorb a single great defeat. At al-Qadisiyyah, around 636, the Arab commander Sa’d ibn Abi Waqqas destroyed the main Sasanian field army and killed its general Rostam Farrokhzad; Ctesiphon fell soon after; and at Nahavand in 642 — remembered in the Arab tradition as the “Victory of Victories” — the last great Persian army was annihilated. Thereafter the provinces fell one by one as Yazdegerd fled before them.

The consequences were civilizational. Zoroastrianism, the state religion, lost its imperial patronage and dwindled over centuries to a minority faith; Persian elites, language, and administrative genius were absorbed into the new caliphate, which they would in turn reshape; and the lands of Iran, conquered as Arab territory, became over generations a heartland of Islam while retaining a fiercely distinct Persian identity. Few conquests have so thoroughly ended one order and seeded another.