Sassanid, Sasanid (both: săs´ənĬd), or Sassanian (săsā´nyən), last dynasty of native rulers to reign in Persia before the Arab conquest. The period of their dominion extended from c.AD 224, when the Parthians were overthrown and the capital, Ctesiphon, was taken, until c.640, when the country fell under the power of the Arabs. The last Sassanid king died a fugitive in 651, but he had been forced to yield Ctesiphon to the Arabs in 636. Under the Sassanids, who revived Achaemenid tradition, Zoroastrianism was reestablished as the state religion. The name of the dynasty was derived from Sassan, an ancestor of the founder of the dynasty, Ardashir I, who took and ruled Ctesiphon (224–40). During his reign and many that followed, war with the Romans occupied much attention. Sassanid persecution of Christians led to wars with Byzantium. Syria and Armenia suffered particularly from invading armies. Ardashir I was succeeded by his son Shapur I, who was victorious over Roman Emperor Valerian and ruled until 272. The next reign of importance was that of Shapur II (309–79), a period of particular significance and glory. Bahram V, ruling 420–38, was defeated by the Emperor Theodosius but succeeded against the White Huns. The Armenians were overwhelmed by Yazdagird II in 451, and their land was overrun by Sassanids under Khosrow I, who reigned 531–79 and who also invaded Syria. Both countries were again overrun by Khosrow II (ruled 590–628), whose conquest of Egypt was the final victorious achievement of the dynasty. The last representative of the family on the throne was Yazdagird III, who began his reign in 632. His struggle against the Arabs ended in the fall of the Sassanid dynasty. See Persia.