By AD 229, Ardashir I was ready to take on Rome: the Romans could not have failed to notice that Persia was now under new management, and Ardashir was determined to strike the first blow - rather than wait to be invaded, as the Parthians had mostly done. He began by destroying Roman power in Mesopotamia and Syria, easy because of their army's poor morale (there'd been mutinies all over the empire).
Rome had had (since AD 222) a new young emperor, Alexander Severus, very conscious of the mistakes and evil reputation of his family. He was, though, completely under the thumb of his mother, Julia Mamaea. The austerity imposed at her insistence to make up for the ludicrous extravagance of previous regimes had not made him popular with the army.
Julia Mamaea, mother of Alexander Severus
Alexander Severus wanted to avoid conflict if he could. He warned Ardashir that his aggression would not be tolerated, and told him to get out of Roman territory, or the Romans would invade, just as Trajan, Verus and Septimius Severus had done.
Ardashir's reply was to ignore this threat, and press on into Armenia and Cappadocia, an area of Asia Minor that hadn't been under Persian rule since the Achaemenids. While Alexander Severus got ready to fight - he now had no alternative - it was (according to Roman sources at least) Ardashir's turn to send the Romans a message: get out of the east completely, and give up the whole of Anatolia.
In AD 231 Alexander attacked in great force, thinking undoubtedly of his famous namesake. After recovering Cappadocia and Syria, three Roman armies advanced into Persia: the northern one through Armenia and Atropatene, the central one (led by Alexander himself) into Mesopotamia, and the southern one straight towards Elymais and Persis. By AD 233 the northern and southern armies had made steady progress, but at Ctesiphon the central army met Ardashir's heavy cavalry (the Savaran knights) for the first time. It was very like Carrhae (53 BC) all over again. According to the historian Herodian:
The Persians trapped the Romans like fish in a net. They shot their arrows from all sides at the encircled legions, and massacred the entire army.Herodian 6.5
Savaran cavalryman - reconstruction
Alexander abandoned his idea of marching on into India. His forces withdrew. There had been no victory for Rome, but this didn't stop Alexander from making a ridiculous speech to the senate back in Rome claiming
I destroyed 218 elephants, 1800 scythed chariots and killed 120,000 of their cavalry.
Ardashir didn't follow up his victory; why not? Possibly he decided that a better policy at this time would be to strengthen the frontiers - or maybe he was tired out after so much campaigning. In any case in AD 240 he retired, and passed the kingship to his son Shapur I - already an experienced commander. Persia was once more united and strong, with a strong disciplined army - and a strong centralized administration - although the feudal Parthian social structure remained in place. But Persia also faced strong enemies to the west (the Romans) and the east (the Kushans - whose friendship Ardashir had bought. But they had not been conquered).
Sasanian king (possibly Ardashir II, but also identified as Khusrau II) on armoured horse with lance and full armour. Taq-i Bustan, near Kermanshah (photo AMW)