Ardashir began as a local ruler in Persis, the southwestern Iranian region that gave the Sasanian dynasty its name, reputedly connected to a family of hereditary custodians of a fire temple dedicated to the goddess Anahita at Istakhr. He expanded his territory through the 210s CE, defeated and killed the last Parthian Arsacid king, Artabanus V, at the Battle of Hormozdgan in 224, and was crowned King of Kings, ending nearly four centuries of Parthian rule and founding the dynasty that would govern Iran until the Arab conquest four centuries later.
Ardashir used explicitly religious imagery to stage this transition. A rock relief at Naqsh-e Rustam shows him receiving the ring of kingship directly from Ahura Mazda, mounted and facing the god as an equal in scale, a visual claim that his rule carried divine sanction rather than resting on conquest alone. His coinage carried Zoroastrian fire altar imagery from early in the reign, a break from the more varied Parthian precedent, and set a pattern his successors continued for the next four centuries.
Ardashir's early campaigns also brought him into conflict with regional rivals within Persis itself before he could turn against the Parthian crown, a sequence of local consolidation historians reconstruct chiefly from his own dynasty's later court histories and from Roman sources describing the new Sasanian state as a far more centralized and assertive neighbor than the Parthians had been.