Exhausted Sasanians fell to Rashidun armies in eighteen years
Between 633 and 651 the Rashidun Caliphate toppled the Sasanian Empire as early Muslim conquests begun under Muhammad in 627/628 reached Iran. A Byzantine–Sasanian war to 628, Khosrow II's execution, and a four-year carousel of ten claimants left Persia hollow before Arab armies arrived.
Khalid ibn al-Walid's 633 push took parts of Asoristan, the Mesopotamian core, before he shifted to the Levant. Sa'd ibn Abi Waqqas's 636 campaign won al-Qadisiyyah, ending Sasanian control west of the Zagros; for six years the mountains were the frontier. By 651 most Iranian urban centres had fallen, Caspian fringe provinces excepted.
Some modern Iranian historians, reading Arab sources against the grain, stress long hard resistance rather than collapse myths. Civil war—the Sasanian Interregnum—mattered as much as battlefield genius. For centuries before, the Euphrates had served as the contested Roman–Persian border, with most fighting and fortification in the hilly north.
The conquest broke a long pattern. Mesopotamia answered to Semitic-speaking rulers for the first time since the Neo-Babylonian Empire fell at Opis in 539 BCE, ending centuries of Achaemenid, Parthian and Sasanian control. It also began the decline of Zoroastrianism, the state faith since about 550 BCE, and persecution drove many adherents to flee. Sasanian society rested on four classes, priests, warriors, secretaries and commoners, with the largest and poorest group carrying the whole tax burden, and Khosrow II's wars pushed taxes beyond what most could pay while wrecking trade and industry. Early Western scholars leaned on the Armenian bishop Sebeos and on Arabic accounts written long afterwards. In 2008 Parvaneh Pourshariati argued that the empire was really a loose confederacy with largely independent Parthian houses, and that the fall of Mesopotamia came later than the conventional 632 to 634.
Source: Muslim conquest of Persia
