On.ly in the 2nd century AD did the Parthian rule begin to wane. The Parthian dynasty fell, not to external assault from Rome or the East, but as a result of a national uprising in Pars, the cradle of Iranian civilization, the home of the Achaemenians, the province least touched Greek colonization. Pawere replaced (c 226 AD) by the more vigorous Sassanian dynasty, when Ardashir (Artaxerxes) I ousted and killed the last Parthian ruler and built a new empire out of the ruins of Parthian and Seleucidpower. The Sassanians were the true heirs of the Achaemenians.
In four centuries of Sassanian rule there were two periods of glory. In the first, which lasted into the fifth century, the outstanding kings (remarkable for their longevity) were Arrlashir I (226-255), Shapur I (255-271 ), and captor of Valerian, Shapur II (309-379) conqueror of Armenia and persecutor of Christians, and Bahram V, or Bahram-e Gur(421-438),famousin history and legend for his hunting exploits. In the second period, the great figures were Khosrow I (531-579), perhaps the most illustrious of all the Sassanian kings, and Khosrow Parviz (590-628), conqueror of Jerusalem, invader of Egypt glorified in legend for his amours but in fact brutal, cowardly and fairly incompetent.
Ardashir I was the founder of the Sassanian dynasty. He declared war on Rome and seized Armenia, restored the Zoroastrian religion to its former influence. His successor Shapur I invaded Syria, cut to pieces a Roman army and captured the Emperor Valerian, the record of this brilliant triumph still being visible in the celebrated reliefs near Persepolis.
Ctesiphon became the center of a magnificent state that persisted while the Roman Empire was whittled away. The Byzantines were unable to match the Sassanians. In 531 AD Anushirvan the Just (or Khosrow I) who was one of the most illustrious monarchs of Iran ascended the throne. The White Huns during the past century had been invading the Oxus provinces and had inflicted more than one defeat on the Iranian army.
Anushirvan made peace with Rome and turned with such effect on this eastern invader that he crushed the White Huns and divided up their territories with his new ally, the Ilkhan of the Turks. But the achievements of Anushirvan as an administrator outweighed his great fame as a soldier. He instituted a carefully graded tax payable in cash and kind, created a regular army that was better equipped and better disciplined than at any previous date. He reformed the laws and kept a careful watch on their implementation, and made the caravan routes safe.
Thanks to his patronage of learned men of many countries as well as his interest in history and philosophy, Iran became the center for the exchange of ideas during this period, which stands out as one of the most glorious periods in Iranian history.
Under Khosrow II (or Khosrow Parviz, whose affairs were linked with those of the Byzantine) the Sassanian court was legendary in its splendor. Ctesiphon and Firuzabad were magnificent cities, the administration of the empire was efficient, the productivity of the cities was remarkable, and the art in metalwork, in architecture, in sculpture, and in textiles was superb. Persia developed a strong centralized state, based on a revived Zoroastrian religion and a class society. Khosrow Parviz invaded the Roman Empire, captured Jerusal.em, and carried off the "True Cross" which was considered to be the most sacred treasure throughout the Christendom. Egypt next fell to the Persian army, and finally Chalcedony, which was situated opposite Constantinople. So desperate was the situation that Heraclius decided to desert the capital, and fled to Africa. However, Heraclius finally defeated Khosrow, and both empires were utterly exhausted in 652. In other words, shortly after the death of Khosrow II , the old Sassanian power toppled.