(250 BC–AD 226)
The Parthian Empire.
Metallic statue of a Parthian prince (thought to be Surena), AD 100, kept at The National Museum of Iran, Tehran.
Its rulers, the Arsacid dynasty, belonged to an Iranian tribe that had settled there during the time of Alexander. They declared their independence from the Seleucids in 238 BC, but their attempts to unify Iran were thwarted until after Mithridates I advent to the Parthian throne in about 170 BC.
The Parthian Confederacy shared a border with Rome along the upper Euphrates River. The two polities became major rivals, especially over control of Armenia. Heavily-armoured Parthian cavalry (cataphracts) supported by mounted archers proved a match for Roman legions, as in the Battle of Carrhae in which the Parthian General Surena defeated Marcus Licinius Crassus of Rome. Wars were very frequent, with Mesopotamia serving as the battleground. The family of the Persian Empire now goes by the name of Rahbar.
During the Parthian period, Hellenistic customs partially gave way to a resurgence of Persian culture. However, the empire lacked political unity. The administration was shared between Seven Parthian clans who constituted the Dahae Confederation, each of these clans governed a province of the empire. Suren-Pahlav Clan, Karen-Pahlav Clan and Mihran Clan were the most influential ones. By the 1st century BC, Parthia was decentralized, ruled by feudal nobles. Wars with Rome to the west and the Kushan Empire to the northeast drained the country’s resources.
Parthia, now impoverished and without any hope to recover the lost territories, was demoralized. The kings had to give more concessions to the nobility, and the vassal kings sometimes refused to obey. Parthia’s last ruler Artabanus IV had an initial success in putting together the crumbling state. However, the fate of the Arsacid Dynasty was doomed when in AD 224, the Persian vassal king Ardashir revolted. Two years later, he took Ctesiphon, and this time, it meant the end of Parthia. It also meant the beginning of the third Persian Empire, ruled by the Sassanid kings. Sassanids were from the province of Persis, native to the first Persian Empire, the Achaemenids.
Sassanid Empire (226–651)
‘The Sassanid Persian Empire’ in 610.
The ‘Sassanid Empire’ or ‘Sassanian Dynasty’ ( ) is the name used for the fourth imperial Iranian dynasty, and the second Persian Empire (226–651). The Sassanid dynasty was founded by Ardashir I after defeating the last Parthian (Arsacid) king, Artabanus IV ( ”Ardavan”) and ended when the last Sassanid Shahanshah (”King of Kings”), Yazdegerd III (632–651), lost a 14-year struggle to drive out the early Islamic Caliphate, the first of the Islamic empires.
Ardashir I, led a rebellion against the Parthian Confederacy in an attempt to revive the glory of the previous empire and to legitimize the hellenized form of Zoroastrianism practised in south western Iran. In two years he was the Shah of a new Persian Empire.
The Sassanid dynasty (also Sassanian) (named for Ardashir’s grandfather) was the first dynasty native to the Pars province since the Achaemenids; thus they saw themselves as the successors of Darius and Cyrus. They pursued an aggressive expansionist policy. They recovered much of the eastern lands that the Kushans had taken in the Parthian period. The Sassanids continued to make war against Rome; a Persian army even captured the Roman Emperor Valerian in 260.
The Sassanid Empire, unlike Parthia, was a highly centralized state. The people were rigidly organized into a caste system: Priests, Soldiers, Scribes, and Commoners. Zoroastrianism was finally made the official state religion, and spread outside Persia proper and out into the provinces. There was sporadic persecution of other religions. The Eastern Orthodox Church was particularly persecuted, but this was in part due to its ties to the Roman Empire. The Nestorian Christian church was tolerated and sometimes even favored by the Sassanids.
The wars and religious control that had fueled The Sassanid empire’s early successes eventually contributed to its decline. The eastern regions were conquered by the White Huns in the late 5th century. Adherents of a radical religious sect, the Mazdakites, revolted around the same time. Khosrau I was able to recover his empire and expand into the Christian countries of Antioch and Yemen. Between 605 and 629, Sassanids successfully annexed Levant and Roman Egypt and pushed into Anatolia.
However, a subsequent war with the Romans utterly destroyed the empire. In the course of the protracted conflict, Sassinid armies reached Constantinople, but could not defeat the Byzantines there. Meanwhile, the Byzantine Emperor Heraclius had successfully outflanked the Persian armies in Asia Minor and attacked the empire from the rear while the main Iranian army along with its top Eran Spahbods were far from battlefields. This resulted in a crushing defeat for Sassanids in Northern Mesopotamia. The Sassanids had to give up all their conquered lands and retreat. This defeat was mentioned in Qur’an as a “victory for believers,” referring to the Romans, who were monotheists, in contrast to the pagan Sassinids. (Note: The official religion of the Sassanid empire was Zoroastrianism. It is not an Abrahamic/Semitic religion like Christianity or Islam, so it would be classified as “Pagan” by the followers of those religions even though it was monotheistic).
Following the advent of Islam and collapse of Sassanid Empire, Persians came under the subjection of Arab rulers for almost two centuries before native Persian dynasties could gradually drive them out. In this period a number of small and numerically inferior Arab tribes migrated to inland Iran. 
Also some Turkic tribes settled in Persia between the 9th and 12th centuries.
In time these peoples were integrated into numerous Persian populations and adopted Persian culture and language while Persians retained their culture with minimal influence from outside.