While the Arsacid Empire of the Parthians inflicted the famous defeat of the Battle of Carrhae in 53 BC, by the second and early-third century AD, the balance of military power had shifted. Between AD 116-200, three campaigns under the emperors Trajan, Marcus Aurelius, and Septimius Severus had inflicted heavy defeats on the Parthians (see also issue III.5).
However, in the opening quarter of the third century, the geopolitical situation changed again. The Arsacid Parthians were replaced as rulers of Iran by a new dynasty. Based in the region of Fars, the heartland of the Achaemenid Empire, the Sasanian dynasty would rise to power and successfully challenge the Arsacids for control. By the AD 230s, Ardashir I had established his control and begun to attack the eastern frontier of Rome. While the Romans had been successful against the Parthians, the Sasanians were far more formidable. Under the first Sasanian Shahanshahs, Ardashir and Shapur I, a far more centralized polity was organized, allowing the Sasanians to deploy their resources far more efficiently. Thus, under the second Shahanshah, Shapur I, the Sasanian military proved to be the most deadly and effective enemy that the Roman Emperors faced during the crises of the third century. Culminating with the defeat and capture of the Emperor Valerian, Shapur’s campaigns, and those of his successors, forced the Roman Empire to reluctantly recognize the Sasanians as their political and military equal
