Founder of the Sasanian dynasty: ruled AD 224- 241
Ardashir I receives the symbol of kingship from Ahura Mazda
When Ardashir defeated the Parthians, he saw himself reconnecting with the Persian empire of the Achaemenids - after the unfortunate 600 year hiatus involving Alexander, the Seleucids, and the Parthians. Like Darius the Great, he feels he owes his power to Ahura Mazda - and he shows himself, like Darius on the Bisitun inscription, trampling his defeated enemy underfoot: for Darius this was the "false Smerdis", Gaumata, for Ardashir it was the Parthian Ardavan V. Notice how similar the king and the god are. The relief was carved into the rock below the Achaemenid tombs at Naqsh-i Rustam, in Fars near Persepolis. Ardashir did not of course know the true identity of his Achaemenid predecessors, but he still wanted to be connected with these former kings, whoever they were.
What was his background?
His origin is not clear. There are numerous unconvincing stories, from which the information derives. He was born around AD 180, in Persis (Fars). His father was called Papag (or Babak or Papak), and may have been of Kurdish descent. Sasan was a hereditary Zoroastrian priest in the fire-temple of Anahita in Istakhr, allegedly descended from the Achaemenids, who may (or may not) have been Papag's father, and hence Ardashir's grandfather. Or was Sasan, as others claim, the actual father of Ardashir, who was later adopted by Papag? We'll never know - but the connection with Sasan, Zoroastrianism and the early kings of Fars was obviously very important to Ardashir, in helping to make out that his rule was genuinely connected with the Persian past.
Papag (father or not) seized the kingship of Persis in around AD 205/6, when he ousted the ruler of Istakhr, capital of Persis (Fars). But when Papag died, his elder son Shapur, not Ardashir, was made king. Ardashir challenged Shapur to fight for his throne - but he died (in suspicious circumstances) before the battle. In AD 208 Ardashir became king of Persis. He duly murdered his other brothers, and crushed any local rulers who might be reluctant to accept him. He refused to recognise the Parthian Ardavan V as his overlord.
The Parthians were right to be worried. Ardashir soon attacked Ctesiphon, with willing support from others who no longer trusted the Parthians - especially the Medes and other peoples from the north-west of the empire.
Ardashir overcomes the Parthians and unites Iran
Ardashir defeated the Parthians in battle three times, finally and decisively at Hormozgan in April AD 224. Two years later, he took Ctesiphon, and was crowned king of kings (AD 226). Ardashir now controlled most of the Iranian plateau. The Parthian nobles rapidly transferred their allegiance to Ardashir. Despite their role in undermining the authority of the Parthian kings, they wanted someone who could stand up to the Romans.
Having imposed his authority on western Iran, in AD 227 Ardashir moved east, where a coalition led by the Arsacid king of Armenia (which included the formidable Kushans of Bactria) was determined to restore Parthian rule. Ardashir defeated them or bought them off, and recovered for Persia the territories of Makran (which included part of north-west Arabia on the other side of the Persian Gulf, now Oman), Sistan, Hyrcania, Margiana and Chorasmia. The Kushans in Bactria and India recognised his authority. Only Armenia remained - it took a ten year war to bring Armenia under his control.
The unity of Iran was an important idea for the Sasanians - it wasn't just a matter of reconquering territory once ruled by the Achaemenids. The land where they lived had been given to the Aryans/Iranians by Ahura Mazda - as demonstrated in the Naqsh-i Rustam relief (above).
The concept of "where the Aryan/Iranians belong" became known as Eranshahr (or Iranshahr) - the home of the Iranians. Other territories ruled by the Sasanians would be called an-Eran (or Aniran: non-Iran). Coins issued by Ardashir were inscribed:
Mazdaean majesty, Ardashir, king of kings of Eran, who is descended from the gods.
Coin of Ardashir I, with a Zoroastrian fire-altar on the reverse
Ardashir built an entirely new capital for himself in Persis, at Gur (Firuzabad). It's remarkable in being completely circular (although several Parthian cities were laid out this way), with the remains of a Zoroastrian fire-temple in the centre:
Ardashir's new circular capital at Firuzabad
He also built himself a magnificent new fortified palace nearby:
Ardashir's palace near Firuzabad, Fars