Arab conquest of the Sasanian Empire

Between about 636 and 651, Arab armies under the early caliphs destroyed the Sasanian Empire, the Persian state that had ruled Iran and Iraq since 224. The decisive battles were Qadisiyyah, fought in Iraq around 636, where the Persian army was beaten and the way to the capital Ctesiphon opened, and Nahavand in 642, after which the Persian field armies could no longer stop the invaders. The last shah, Yazdegerd III, fled from province to province and was killed near Merv, in what is now Turkmenistan, in 651.²³⁴

The Sasanians were in poor shape to resist. The long war with the Byzantines that ended in 628 had drained their treasury and their manpower, and the shah Khosrow II had been overthrown and killed at the end of it. A string of short-lived rulers and rival generals followed, and the empire had barely begun to recover when the first Arab raiders arrived. The Arab armies came out of the conquests set off by the unification of Arabia under the new caliphate after Muhammad's death in 632, and they were mobile, motivated and led by effective commanders such as Khalid ibn al-Walid and Sa'd ibn Abi Waqqas.¹⁵

Iran did not turn Arab or Muslim overnight. Conversion was slow and took centuries, Zoroastrian communities lasted for generations, and Persian remained the language of the land and eventually returned to prominence in writing. What changed immediately was the end of the old imperial order: the Zoroastrian state church lost its protector, Iran's tax and administrative systems were absorbed into the caliphate, and Persian officials, scholars and ideas flowed into the new Islamic world, especially under the Abbasids.¹³⁵