The beginnings of the Sassanian dynasty (alternatively Sasanian or Sasanid) are to be found in the southwest Iranian state of Pars, known as Persis to the Greeks (its modern name is Fars). Sometime in the first decade of the 3rd century CE, a king by the name of Papak came into power by seizing the throne of Pars. After unifying the region under his command, Papak waged a difficult war against the central Parthian government, but died before the conflict was resolved. Papak's son Ardashir inherited the throne in 216, and continued the campaign against the Parthian Empire until 224. In this year, a coalition force under Ardashir met the army of the last Parthian ruler, Artabanus V. The Parthians lost the day, with Artabanus killed in the battle. Over the next years Ardashir unified all of Iran under his rule, and appointed members of his family to control provinces bordering Syria in the west.1
According to one tradition, the Sassanians were named after a 1st century prince or king by the name of Sasan, from whom Papak claimed to be descended. An alternative legend claims that Sasan was the name of a shepherd in Papak's kingdom. Papak dreamed the son of this peasant would become a great emperor, and in order to participate in this providence, he gave his own daughter to Sasan. The legend has it that from this union Ardashir I was born.2
By the end of Ardashir's reign (241) the Sassanian Empire stretched from Sogdiana in the north to the Mazun in the Arabian south, from the Indus River Valley in the east to the borders of Roman Syria in the west. But in terms of its size, the Sassanian Empire was constantly in flux throughout its history, sometimes growing, sometimes lessening. This was in reaction to the pressures applied to its borders by the Romans and Byzantines in the west, and the Kushans and then Hephthalites in the east.
Under Sassanian rule, Iranian culture experienced something of a renaissance. The Sassanian monarchs hoped to destroy the remaining vestiges of Greek culture that had lingered since the Seleucid era, and supported the development of native art, architecture, and literature. Zoroastrianism became the state religion, and in accordance with Zoroastrian teachings, society was divided into four separate classes: priest, warriors, scribes and peasants. The priestly class grew to carry immense political influence, and formed the upper stratum of Sassanian society alongside the royalty.
During the mid-third century, the Sassanian conquest of the western portion of the Kushan Empire disrupted the flow of trade from Central Asia trade into the Middle East, as Indian and Sogdian merchants searched out safer routes.3 Once stability returned to the Kushano-Sassanian provinces, Iranian branches of the silk route began to see an increase in trade once more. Luxury items such as ceramics, glass, and textiles traveled both east and west, with Sassanian merchants serving as middlemen between the Syrian traders in the west and Sogdians in the east.
By the beginning of the seventh century, the Sassanian government began to demonstrate signs of decline. Chosroes II (ruled 591-628) garnered a reputation for squandering resources and tax revenues in lavish displays of wealth. Near the end of his reign Chosroes initiated an offensive against the Byzantine Emperor Heraclius, who had defeated the Sassanians in a decisive battle at Issus and Halys in 622. Initially his campaign saw some success, and in 626 Chosroes' army besieged Constantinople. But their success was short-lived, as Byzantine forces soon drove them back, seizing territory as they advanced in to Sassanian lands. In 628 a revolution broke out, and Chosroes was killed by his own minister. Weakened by years of warfare, heavy taxation, and poor economic conditions, the Sassanians were in little shape to defend their borders when the Arabs invade in 637 at Al Quadesiya. The Sassanian government collapsed soon afterwards, leaving Iran ripe for occupation by the Arab invaders.
(1) B. A. Litvinsky, "The Rise of Sasanian Iran," from History of Civilizations of Central Asia, Vol. II (Paris: UNESCO, 1994), pp. 475-6.
(2) "Sasan," Encyclopædia Britannica Online
(3) Op. Cit., p. 484.