The empire was founded by Ardashir I, who claimed descent from the Achaemenid dynasty of antiquity. In 224 he overthrew the Arsacid Parthians and established a new Persian Empire that would rule the region for the next seven hundred years. Before long the Sassanids had established the same rivalry with Rome that their predeccessors had, resulting in numerous conflicts between the two empires over the centuries.
From 602-628 Persia fought a long and bloody war with Rome, provoked by the usurpation and assassination of Shah Khusrau II's friend and ally, the Emperor Maurice. The war critically weakened both empires, leaving them vulnerable to the emerging Talibid Caliphate. However, in 632 the Emperor Heraclius converted to Islam, allying with the Caliphate and succeeding in restraining its expansionist tendencies.
In 634 Persia attacked the Beni Lakhm, an Arab people who previously been Persian vassals but had recently rebelled and deserted to the Caliphate. When the Caliphate tried to assist the Lakhmids it only succeeded in provoking a full-scale war, that eventually dragged in Rome and Axum and resulted in a devastating Persian defeat. Mesopotamia was lost to Rome, and Shah Yazdegered III was captured, which would trigger decades worth of civil wars between rival branches of the dynasty fighting for the throne.
Yazdegerd's young son Firuz Nirvasita escaped to India, where he founded the Indo-Sassanid Empire. His son Narsieh eventually recaptured Persia itself from the usurpers, breathing new life into the Sassanid state.
The Sassanid state emerged from these wars badly weakened, and made efforts in the following years to maintain peace with its neighbours, though it was unable to prevent several of the Khorasanian provinces from being conquered by the Oghuz Turks. Eventually, in 854, Kavadh III agreed to allow Muslim missionaries to proselytize in the region west of the Zagros Mountains as the price of a peace treaty with the Roman Empire. Kavadh's political enemies furiously denounced this act and he was assassinated the following year, plunging Iran into another round of civil wars in which most of the remaining members of the dynasty were killed.
By the year 929 there were only two male claimants to the throne left alive - the brothers Ardashir VII and Shapur VI, who had their courts in Susa and Isfahan respectively. The remainder of the country was in the hands of ambitious generals and nobles, busy carving out their own kingdoms from the wreckage. In that year Shapur was assassinated by agents of Ardashir, who himself died from malaria a few months later, and with their demise the Sassanid Empire came to an end.
The many successor states eventually consolidated themselves into the Five Western Kingdoms of western Iran, and the Shansabanid Empire of the east. The country as a whole would not be reunited until the 14th century under the Mehranid dynasty.