As opposed to the Arsacids, the Sasanians, like their Achaemenid 'ancestors', tell a great deal about their notions of government, their public appearances and their political aspirations in both the domestic and foreign spheres. All lands of the former Parthian empire, except for Armenia, came under Sasanian control during the reign of the founder of the dynasty, Ardashir. Yazdgerd III was made king by Rustam's aristocratic party, thus becoming the Sasanians' last ruler. It was a decidedly Iranian attitude that characterised the Sasanian image of the ruler and his qualities. The late Sasanian period was altogether a time of literary flowering, much of it commissioned or sponsored by the royal court. The Sasanian empire was also characterised by the magnitude and diversity of its religious groups and communities. Whereas the fourth century was characterised by numerous military conflicts between the superpowers Iran and Byzantium.

The rise of the Sassanians is better documented than subsequent history, but even so there are conflicting versions of the origin of Ardashir. The first capital of Ardashir in Fars most likely was at Firuzabad, then known as Gor, although later in the reign of Ardashir it received the name Ardashir-khwarreh 'the glory of A', probably in honour of his victories over the Parthians. The Sassanians respected and feared the mighty Roman Empire and continued to designate the later Byzantine state by the same name, although they knew that different peoples lived in the enemy state and served in its armies. The Romans too learned to respect the Sassanians more than they had the weaker Parthians. Perhaps the most significant changes in outlook, culture and society in the third and fourth centuries, both in the Roman and the Sassanian worlds, were the changes in religion which marked the end of the old 'pagan' religions and the flowering of 'universal' religions.