Ardashir I (ärdäshēr´) [another form of Artaxerxes], d. 240, king of Persia (226?–240). He overthrew the last Parthian king, Artabanus IV, entered Ctesiphon, and reunited Persia out of the confusion of Seleucid decline. He established the strong Sassanid, or Sassanian, dynasty and reconquered the old eastern territories. Ardashir established Zoroastrianism as the state religion and gave much power to the priestly caste. His move against Mesopotamia, Armenia, and Cappadocia caused the Roman emperor Alexander Severus to campaign against him. A great battle in 232 cost both armies heavy losses. It was Alexander who had to retire, and though Alexander celebrated a triumph in Rome, Ardashir took Armenia, and Persian power was firmly established. He is sometimes called Ardashir Papakan, for his father, Papak. Shapur I succeeded him.