The Sassanid era, encompassing the length of the Late Antiquity period, is considered to be one of Iran's most important and influential historical periods. In many ways the Sassanid period witnessed the highest achievement of Persian civilization, and constituted the last great Iranian Empire before the Muslim conquest and adoption of Islam. Persia influenced Roman civilization considerably during the Sassanids' times,^[3] and the Romans reserved for the Sassanid Persians alone the status of equals, exemplified in the letters written by the Roman Emperor to the Persian Shahanshah, which were addressed to "my brother." Their cultural influence extended far beyond the empire's territorial borders, reaching as far as Western Europe,^[4] Africa,^[5] China and India^[6] and played a prominent role in the formation of both European and Asiatic medieval art.^[7]

This influence carried forward to the early Islamic world with the Muslim conquest of Iran, especially the dynasty's unique, aristocratic culture.^[8] Zarinkoob even goes to the extent of claiming that much of what later became known as Islamic culture, architecture, writing and other skills was borrowed mainly from the Sassanid Persians and propagated throughout the broader Muslim world.^[9]

The last years of the Arsacids, and the rise of the Sassanids (or Sassanians) are not well known because of scanty and conflicting information in the sources.^[10] The Sassanid Dynasty was established by Ardashir I, a descendant of a line of the priests of goddess Anahita in Istakhr, who at the beginning of the third century had acquired the governorship of Persis (modern Persian Fars). There is, however, a problem about the origin of Ardashir and his relationship to his purported ancestors, Sassan and Pāpağ (Babak). It is unclear whether he was a natural or adopted son of Pāpağ, and whether Sasan was the eponymous founder of the dynasty or Ardashir's real father or father-in-law. In general, sources are not consistent as far as the relationships between the early Sassanids (Sassan, Babak, Ardashir and Shapur) are concerned.^[11]

Babak was originally the ruler of a small town called Kheir, but had managed by 200 to depose Gocihr, the last king of the Bazrangids (the local rulers of Persis as a client of the Arsacids) and appointed himself as the new ruler. His mother, Rodhagh, was the daughter of the provincial governor of Persis. Babak and his eldest son Shapur managed to expand their power over all of Persis. The subsequent events are unclear, due to the sketchy nature of the sources. It is however certain that following the death of Babak, Ardashir who at the time was the governor of Darabgird, got involved in a power struggle of his own with his elder brother Shapur. The sources tell us that Shapur, leaving for a meeting with his brother, was killed when the roof of a building collapsed on him; by 208 over the protests of his other brothers, who were put to death, Ardashir declared himself ruler of Persis.^[12]

At this point, Ardashir moved his capital further to the south of Persis and founded Ardashir-Khwarrah (formerly Gur, modern day Firouzabad). The city, well supported by high mountains and easily defendable through narrow passes, became the center of Ardashir's efforts to gain more power. The city was surrounded by a high, circular wall, probably copied from that of Darabgird, and on the north-side included a large palace, remains of which still survive. After establishing his rule over Persis, Ardashir I rapidly extended his territory, demanding fealty from the local princes of Fars, and gaining control over the neighboring provinces of Kerman, Isfahan, Susiana, and Mesene. This expansion quickly came to the attention of Artabanus IV, the Parthian king, who initially ordered the governor of Khuzestan to march against Ardashir in 224, but this ended up in a major victory for Ardashir. Artabanus himself marched a second time against Ardashir I in 224, but, after their armies clashed at Hormozgan, where Artabanus was killed, Ardashir I went on to invade the western provinces of the now defunct Parthian Empire.^[13]

Factors that aided the rise to supremacy of the Sassanids were the Artabanus-Vologases dynastic struggle for the Parthian throne, which probably allowed Ardashir to consolidate his authority in the south with little or no interference from the Parthians, and the geography of the Fars province, which separated it from the rest of Iran.^[14] Crowned in 224 at Ctesiphon as the sole ruler of Persia, Ardashir took the title Shahanshah, or "King of Kings" (the inscriptions mention Adhur-Anahid as his "Queen of Queens", but her relationship with Ardashir is not established), bringing the 400-year-old Parthian Empire to an end and beginning four centuries of Sassanid rule.^[15]

Ardashir I's son Shapur I continued the expansion of the empire, conquering Bactria and the western portion of the Kushan Empire, while leading several campaigns against Rome. Invading Roman Mesopotamia, Shapur I captured Carrhae and Nisibis, but in 243 the Roman general Timesitheus defeated the Persians at Rhesaina and regained the lost territories.^[17] The emperor Gordian III's (238–244) subsequent advance down the Euphrates was defeated at Meshike (244), leading to Gordian's murder by his own troops and enabling Shapur to conclude a highly advantageous peace treaty with the new emperor Philip the Arab, by which he secured the immediate payment of 500,000 denari and further annual payments. Shapur soon resumed the war, defeated the Romans at Barbalissos (252), and then probably took and plundered Antioch.^[18] Roman counter-attacks under the emperor Valerian ended in disaster when the Roman army was defeated and besieged at Edessa and Valerian was captured by Shapur, remaining his prisoner for the rest of his life. Shapur celebrated his victory by carving the impressive rock reliefs in Naqsh-e Rostam and Bishapur, as well as a monumental inscription in Persian and Greek in the vicinity of Persepolis. He exploited his success by advancing into Anatolia (260), but withdrew in disarray after defeats at the hands of the Romans and their Palmyrene ally Odaenathus, suffering the capture of his harem and the loss of all the Roman territories he had occupied.^[19]

Succeeding Bahram III (who ruled briefly in 293), Narseh embarked on another war with the Romans. After an early success against the Emperor Galerius near Callinicum on the Euphrates in 296, Narseh was decisively defeated in an ambush while he was with his harem in Armenia in 298. In the treaty that concluded this war, the Sassanids ceded five provinces east of the Tigris and agreed not to interfere in the affairs of Armenia and Georgia.^[22] Following this crushing defeat, Narseh resigned in 301 and died in grief a year later. Narseh's son Hormizd II suppressed revolts in Sistan and Kushan, but was unable to control the nobles; he was killed by Bedouins while hunting in 309.

Following Hormizd II's death, Arabs from the south started to ravage and plunder the southern cities of the empire, even attacking the province of Fars, the birthplace of the Sassanid kings. Meanwhile, Persian nobles killed Hormizd II's eldest son, blinded the second, and imprisoned the third (who later escaped to Roman territory). The throne was reserved for Shapur II, the unborn child of one of Hormizd II's wives who was crowned in utero: the crown was placed upon his mother's stomach.^[23] During his youth the empire was controlled by his mother and the nobles. Upon Shapur II's coming of age, he assumed power and quickly proved to be an active and effective ruler.

Shapur II pursued a harsh religious policy. Under his reign the collection of the Avesta, the sacred texts of Zoroastrianism, was completed, heresy and apostasy were punished, and Christians were persecuted. The latter was a reaction against the Christianization of the Roman Empire by Constantine the Great. Shapur II, like Shapur I, was amicable towards Jews, who lived in relative freedom and gained many advantages in his period. At the time of Shapur's death, the Persian Empire was stronger than ever, with its enemies to the east pacified and Armenia under Persian control.^[26]

Bahram IV's son Yazdegerd I (399–421) is often compared to Constantine I. Like him, he was powerful both physically and diplomatically. Much like his Roman counterpart, Yazdegerd I was opportunistic. Like Constantine the Great, Yazdgerd I practiced religious tolerance and provided freedom for the rise of religious minorities. He stopped the persecution against the Christians and even punished nobles and priests who persecuted them. His reign marked a relatively peaceful era. He made lasting peace with the Romans and even took the young Theodosius II (408–450) under his guardianship. He also married a Jewish princess who bore him a son called Narsi.

Yazdegerd I's successor was his son Bahram V (421–438), one of the most well-known Sassanid kings and the hero of many myths. These myths persisted even after the destruction of the Sassanid empire by the Arabs. Bahram V, better known as Bahram-e Gur, gained the crown after Yazdgerd I's sudden death (or assassination) against the opposition of the grandees with the help of al-Mundhir, the Arabic dynast of al-Hirah. Bahram V's mother was Soshandukht, the daughter of the Jewish Exilarch. In 427 he crushed an invasion in the east by the nomadic Hephthalites, extending his influence into Central Asia, where his portrait survived for centuries on the coinage of Bukhara (in modern Uzbekistan). Bahram V deposed the vassal King of the Persian part of Armenia and made it a province.

Bahram V is a great favorite in Persian tradition, which relates many stories of his valor and beauty, of his victories over the Romans, Turks, Indians and Africans, and of his adventures in hunting and in love; he is called Bahram-e Gur, Gur meaning Onager, on account of his love for hunting and, in particular, hunting onagers. He symbolized a king in the height of a golden age. He had won his crown by competing with his brother and spent time fighting foreign enemies, but mostly kept himself amused by hunting and court parties with his famous band of ladies and courtiers. He embodied royal prosperity. During his time the best pieces of Sassanid literature were written, notable pieces of Sassanid music were composed, and sports such as polo became royal pastimes, a tradition that continues to this day in many kingdoms.^[29]

At the beginning of his reign, Yazdegerd II gathered a mixed army of various nations, including his Indian allies, and attacked the Eastern Roman Empire in 441, but peace was soon restored after small-scale fighting. He then gathered his forces in Neishabur in 443 and launched a prolonged campaign against the Kidarites. Finally after a number of battles, he crushed the Kidarites and drove them out beyond Oxus river in 450.^[31]

During his eastern campaign, Yazdegerd II grew suspicious of the Christians in the army and expelled them all from the governing body and army. He then persecuted the Christians and, to a much lesser extent, the Jews.^[32] In order to reestablish Zoroastrianism in Armenia, he crushed an uprising of Armenian Christians at the Battle of Vartanantz in 451. The Armenians, however, remained primarily Christian. In his later years, he was engaged yet again with Kidarites until his death in 457. Hormizd III (457–459), younger son of Yazdegerd II, ascended to the throne. During his short rule, he continually fought with his elder brother Peroz, who had the support of nobility,^[32] and with the Hephthalites in Bactria. He was killed by his brother Peroz in 459.

In the beginning of the 5th century, the Hephthalites (White Huns), along with other nomadic groups, attacked Persia. At first Bahram V and Yazdegerd II inflicted decisive defeats against them and drove them back eastward. The Huns returned at the end of 5th century and defeated Peroz I (457–484) in 483. Following this victory the Huns invaded and plundered parts of eastern Persia for two years. They exacted heavy tribute for some years thereafter.

These attacks brought instability and chaos to the kingdom. Peroz I tried again to drive out the Hephthalites, but on the way to Herat, he and his army were trapped by the Huns in the desert; Peroz I was killed, and his army was wiped out. After this victory the Hephthalites advanced forward to the city of Herat, throwing the empire into chaos. Eventually, a noble Persian from the old family of Karen, Zarmihr (or Sokhra), restored some degree of order. He raised Balash, one of Peroz I's brothers, to the throne, although the Hunnic threat persisted until the reign of Khosrau I. Balash (484–488) was a mild and generous monarch, who made concessions to the Christians; however, he took no action against the empire's enemies, particularly, the White Huns. Balash, after a reign of four years, was blinded and deposed (attributed to magnates), and his nephew Kavadh I was raised to the throne.

Kavadh I (488–531) was an energetic and reformist ruler. Kavadh I gave his support to the communistic sect founded by Mazdak, son of Bamdad, who demanded that the rich should divide their wives and their wealth with the poor. His intention evidently was, by adopting the doctrine of the Mazdakites, to break the influence of the magnates and the growing aristocracy. These reforms led to his being deposed and imprisoned in the "Castle of Oblivion" (Lethe) in Susa, and his younger brother Jamasp (Zamaspes) was raised to the throne in 496. Kavadh I, however, escaped in 498 and was given refuge by the White Hun king.

Djamasp (496–498) was installed on the Sassanid throne upon the deposition of Kavadh I by members of the nobility. Djamasp was a good and kind king, and he reduced taxes in order to relieve the peasants and the poor. He was also an adherent of the mainstream Mazdean religion, diversions from which had cost Kavadh I his throne and freedom. His reign soon ended when Kavadh I, at the head of a large army granted to him by the Hephthalite king, returned to the empire's capital. Djamasp stepped down from his position and restored the throne to his brother. No further mention of Djamasp is made after the restoration of Kavadh I, but it is widely believed that he was treated favorably at the court of his brother.^[29]

Although hugely successful at first glance, Khosrau II's campaign had in fact overextended the Persian army and overtaxed the people. The Byzantine emperor Heraclius (610–641) drew on all his diminished and devastated empire's remaining resources, reorganised his armies and mounted a remarkable counter-offensive. Between 622 and 627 he campaigned against the Persians in Anatolia and the Caucasus, winning a string of victories against Persian forces under Khosrau, Shahrbaraz, Shahin and Shahraplakan, sacking the great Zoroastrian temple at Ganzak and securing assistance from the Khazars and Western Turkic Khaganate. In 626 Constantinople was besieged by Slavic and Avar forces which were supported by a Persian army under Shahrbaraz on the far side of the Bosphorus, but attempts to ferry the Persians across were blocked by the Byzantine fleet and the siege ended in failure. In 627-8 Heraclius mounted a winter invasion of Mesopotamia and, despite the departure of his Khazar allies, defeated a Persian army commanded by Rhahzadh in the Battle of Nineveh. He then marched down the Tigris, devastating the country and sacking Khosrau's palace of Dastagerd. He was prevented from attacking Ctesiphon by the destruction of the bridges on the Nahrawan Canal and conducted further raids before withdrawing up the Diyala into north-western Iran.^[36]

The impact of Heraclius's victories, the devastation of the richest territories of the Sassanid Empire and the humiliating destruction of high-profile targets such as Ganzak and Dastagerd fatally undermined Khosrau's prestige and his support among the Persian aristocracy, and early in 628 he was overthrown and murdered by his son Kavadh II (628), who immediately brought an end to the war, agreeing to withdraw from all occupied territories. In 629 AD Heraclius restored the True Cross to Jerusalem in a majestic ceremony.^[36] Kavadh died within months and chaos and civil war followed. Over a period of four years and five successive kings, including two daughters of Khosrau II and spahbod Shahrbaraz, the Sassanid Empire weakened considerably. The power of the central authority passed into the hands of the generals. It would take several years for a strong king to emerge from a series of coups, and the Sassanids never had time to recover fully.^[35]

In the spring of 632, a grandson of Khosrau I who had lived in hiding, Yazdegerd III, ascended the throne. The same year, the first raiders from the Arab tribes, newly united by Islam, arrived in Persian territory. Years of warfare had exhausted both the Byzantines and the Persians. The Sassanids were further weakened by economic decline, heavy taxation, religious unrest, rigid social stratification, the increasing power of the provincial landholders, and a rapid turnover of rulers. These factors facilitated the Islamic conquest of Persia.

The Sassanids never mounted a truly effective resistance to the pressure applied by the initial Arab armies. Yazdegerd was a boy at the mercy of his advisers and incapable of uniting a vast country crumbling into small feudal kingdoms, despite the fact that the Byzantines, under similar pressure from the newly expansive Arabs, no longer threatened. Caliph Abu Bakr's brilliant commander Khalid ibn Walid moved to capture Iraq in series of lightning battles. Redeployed to the Syrian front against the Byzantines in June 634, Khalid's successor in Iraq failed him and Muslims were defeated in the Battle of the Bridge in 634 which resulted in a Sassanid victory, however the Arab threat did not stop there and reappeared shortly from the disciplined armies of Khalid ibn Walid, once one of Muhammad's chosen companions-in-arms and leader of the Arab army. Under the Caliph `Umar ibn al-Khattāb, a Muslim army defeated a larger Persian force lead by general Rostam Farrokhzad at the plains of al-Qādisiyyah in 637 and besieged Ctesiphon. Ctesiphon fell after a prolonged siege. Yazdgerd fled eastward from Ctesiphon, leaving behind him most of the Empire's vast treasury. The Arabs captured Ctesiphon shortly afterward, leaving the Sassanid government strapped for funds and acquiring a powerful financial resource for their own use. A number of Sassanid governors attempted to combine their forces to throw back the invaders, but the effort was crippled by the lack of a strong central authority, and the governors were defeated at the Battle of Nihawānd; the empire, with its military command structure non-existent, its non-noble troop levies decimated, its financial resources effectively destroyed, and the Asawaran (Azatan) knightly caste destroyed piecemeal, was now utterly helpless in the face of the invaders.

Upon hearing the defeat in Nihawānd, Yazdgerd along with most of Persian nobilities fled further inland to the eastern province of Khorasan. He was assassinated by a miller in Merv in late 651 while the rest of the nobles settled in central Asia where they contributed greatly in spreading Persian culture and language in those regions and the establishment of the first native Iranian Islamic dynasty, the Samanid dynasty, which sought to revive and resuscitate Sassanid traditions and culture after the invasion of Islam.

The abrupt fall of Sassanid Empire was completed in a period of five years, and most of its territory was absorbed into the Islamic caliphate; however, many Iranian cities resisted and fought against the invaders several times. Cities such as Rayy, Isfahan and Hamadan were exterminated thrice by Islamic caliphates in order to suppress revolts.^[37] The local population, initially under little pressure to convert to Islam, remained as dhimmi subjects of the Muslim state and paid a poll tax (jizya),^[38]. Conversion of the Persian population to Islam would take place gradually, particularly as Persian-speaking elites attempted to gain positions of prestige under the Abbasid Caliphate. Invaders destroyed the Academy of Gundishapur and its library, burning piles of books. Most Sassanid records and literary works were destroyed. A few that escaped this fate were later translated into Arabic and later to Modern Persian.^[29] During the Islamic invasion many Iranian cities were destroyed or deserted, palaces and bridges were ruined and many magnificent imperial Persian gardens were burned to the ground.^[39] Persian poets such as Ferdowsi lamented the downfall of the Sassanids in their work:

It is believed that the following dynasties and religious leaders have ancestors among the Sassanian rulers:

• The Bawendiyes dynasty (665 - 1006) of Mazenderan, descendant of Kavadh I. ^[40]
• The Sassanids of Georgia, also known as Khosroides (265 - 570), who sprang from Shapur I. ^[41]
• The Gavparehids of Tabaristan (647 - 1597), children of Djamasp. ^[42]
• The Ghaznavids (977 - 1187), with a Persian ancestor: Yazdgerd III. ^[43]
• The Shahs of Shirwan (1100 - 1382) from Hormizd IV's line. ^[44]
• Bahá'u'lláh (1817-1892) founder of the Bahai Faith ,with a Persian ancestor: Yazdgerd III.