The Sassanian dynasty (A.D. 227-651)
"The immediate causes which brought about the overthrow of the Parthian kingdom and the establishment of the dynasty of Sassan in its stead are not known. The new dynasty of the Sassanids was a more genuine representative of the civilized Iranian race than the Parthian Arsacidæ, especially as far as religion was concerned. 
The founder of the Sassanian dynasty, Ardashir Papakan (Artaxerxes, son of Papak), was born at Persis, in central Iran; his family claimed descent from a mythical ancestor, Sassan, and he was therefore of the priestly caste. Babek, the father of Ardashir, seems to have founded a small kingdom at Persis, and to have annexed the territories of other lesser princes, thus gradually encroaching on various Parthian provinces. 
Vologeses V, the last king of the Arsacid dynasty of Parthia, declared war against the rising chief, but was defeated and put to death by Ardashir A.D. 227. Thus the Parthian Empire passed into the hands of the Sassanian dynasty. The surviving Arsacids fled to India, and all the provinces accepted Ardashir's rule without resistance. It was in fact the beginning of a new and religious movement, the new dynasty being looked upon as the true and genuine successor of the old and noble Achæmenian dynasty, and of the Zoroastrian religion."
One of the first acts of Ardashir was to send an embassy to Rome demanding that the whole of Western Asia should be ceded to him. Soon afterwards, in 230, he sought to regain the lost provinces of Mesopotamia by force of arms. The emperor, Alexander Severus, opposed the advance of Ardashir's army, but was only partly successful. Ardashir devoted the remaining years of his reign to founding new towns, schools, and temples and to reorganizing the judicial system of the courts and the army. Everywhere were evidences of a new development of the true Iranian spirit; and it was not long before the Persian nation deemed itself sufficiently strong once more to enforce its old claims to the sovereignty of Western Asia. 
Sapor I, the son of Ardashir, who reigned from 240 to 273, renewed the war with Rome, first against Gordian, then against Valerian. The latter emperor was treacherously seized at a conference in 260, and spent the rest of his life in a Persian prison subject to most barbarous ill-usage. Sapor then conquered Syria and destroyed Antioch, but was finally driven back by Odenathus, King of Palmyra. After the death of Odenathus the war was continued by his widow, Zenobia, who was so elated by her success that she attempted to found an independent Syrian empire under the leadership of Palmyra, but was defeated and taken prisoner by the Romans under Aurelian.
Hormuz and Bahram I
The Third Sassanid king, Hormuz, reigned only one year; his successor, Bahram I (274-77), continued the war with Zenobia and afterwards with Aurelian. But this war terminated, without any result, at the death of Aurelian, in 275. During this period, the revival of the Zoroastrian religion became a movement of great importance. Having attained ascendancy in Persia under the early Sassanid kings, it grew very intolerant, persecuting alike heathen and Christian. It first turned against Mani, the founder of Manichæism, and his followers, under Bahram I. Mani himself, at first in favour at the Persian Court, was crucified about the year 275. 
Under the next king, Bahram II (277-94), Persia suffered severe reverses from the Roman Emperor Carus, the capital city, Ctesiphon, even falling into the hands of the Romans. 
Bahram III and Narsi I
Bahram III, son of Bahram II, reigned only eight months, and was succeeded by his younger brother, Narsi I, who renewed the war with Rome with Disastrous results. 
Hormus II and Sapor II
He was succeeded by his son, Hormuz II (303-10), and he, again, by Sapor II (310-81). It was in the latter reign that the Christians in Persia suffered serious persecution. 
During the early years of Sapor II the Christian religion received formal recognition from Constantine and there is no doubt that this identification of the Church with the Roman Empire was the chief cause of its disfavour in Persia. Moreover, there is evidence that Christianity had spread widely in the Persian dominions, and every Christian was suspected of disaffection towards the Persian king and secret attachment to the Roman Empire, the more so because even the Persian-speaking Christians employed the Syriac language in their worship. Probably this feeling of suspicion was increased by the letter which Constantine wrote to Sapor (Theod., "H.E.", I, xxv), asking protection for the Christians resident in Persia. (See III, below.) To this period belongs Aphraates, a converted Persian noble, a writer of homilies. When Constantine was dead, and the Magi had attained complete ascendancy over the Persian king, a persecution ensued which was far more severe than any of those of the Roman Emperors.
This attack upon the Christians was but part of Sapor's anti-Western policy. In 350 he openly declared war against Rome, and marched on Syria. The first important action was the siege of Nisibis, where the famous Jacob, founder of the school of Nisibis, was then bishop. The siege lasted seventy days, and then the Persians having build a dam across the River Mygdonius, the waters broke down the wall. The siege was unsuccessful, however, and the campaign ended in a truce. Julian, who became emperor in 362, determined to invade the dominions of Sapor. In March, 363, he set out from Antioch to march towards Carræ. From the latter point two roads led to Persia: one through Nisibis to the Tigris, the other turning south along the Euphrates and then crossing the lower Tigris. Julian chose the second of these and, passing through Callinicum, Carchemish, and Zaitham, reached the Persian capital, Ctesiphon, where he was met with proposals of peace from Sapor, but refused them. After crossing the Tigris, he burned his ships to prevent their falling into the hands of the enemy; but the result was something like a panic amongst his followers. Supplies ran short, and the army entered the desert, where it seems to have lost its way. There had been no battle as yet, but almost daily skirmishes with the light-armed Persian cavalry. In one of these skirmishes Julian was slain by a javelin, whether thrown by one of the enemy or by one of his own followers has never been known. The soldiers at once elected Jovian, one of Julian's generals, and he began his reign by making a thirty years' truce with Persia. The Persians were to supply guides and food for the retreat, while the Romans promised to surrender Nisibis and give up their protectorate over Armenia and Iberia, which became Persian provinces. The surrender of Nisibis put an end to the school established there by Jacob, but his disciple Ephraim removed to Edessa, and there reestablished the school, so that Edessa became one more the centre of Syriac intellectual life. With this school must be connected the older Syriac martyrologies, and many of the Syriac translations and editions of Greek church manuals, canons, and theological writers. Thus were preserved Syriac versions of many important works, the original Greek of which is lost.
In spite of this thirty years' truce, the Persians for a time kept up a petty warfare, the Romans acting on the defensive. But as age rendered Sapor helpless, this warfare died out. Sapor died in 380, at the age of seventy; being a posthumous son, he had spent his whole life on the throne.
Sapor III and Bahram IV
During the reigns of Sapor III and Bahram IV Persia remained at peace. In 379 the Emperor Theodosius the Great received an embassy from Persia proposing friendly relations. This was mainly due to the fact that the Persians had difficulties on their northern and eastern frontiers, and wished to have their hands free in the west. Incidentally, it may be noted that the flourishing period of the "middle school", under the leadership of Dorotheus, and the spread of monasticism through Persia and Mesopotamia were contemporary with the disastrous expedition and peace of Jovian. The great bishop, Jacob of Nisibis, forms a connecting ling with Sapor II; he encouraged Nisibis in its first resistance to the army of Sapor; his school at Nisibis was modelled on that of Diodorus at Antioch, and he was the patron and benefactor of the monastery founded by Awgin on Mont Izla.
In 399 Bahram IV was succeeded by his younger brother Yezdegerd (399-420). Early in this reign Maruthas, Bishop of Maiperkat, in Mesopotamia, was employed by the Roman emperor as envoy to the Persian Court. Maruthas quickly gained great influence over the Persian king, to the annoyance of the Zoroastrian magi, and Yezdegerd allowed the free spread of Christianity in Persia and the building of churches. Nisibis once more became a Christian city. The Persian Church at this period seems to have received, under Maruthas, the more developed organization under which it lived until the time of the Mohammedan conquest. (See III, below.) Later in the reign of Yezdegerd, the Persian bishop, Abdas of Susa, was associated with Maruthas, and, by his impetuosity, put an end to the good relations between the Persian king and the Christians. Abdas destroyed one of the fire temples of the Zoroastrians; complaint was made to the king, and the bishop was ordered to restore the building and make good all damage that he had committed. Abdas refused to rebuild a heathen temple at his own expense. The result was that orders were issued for the destruction of all churches, and these were carried out by the Zoroastrians, who had regarded with great envy the royal favour extended to Maruthas and his co-religionists. Before long the destruction of churches developed into a general persecution, in which Abdas was one of the first martyrs. When Yezdegerd died in 420, and was succeeded by his son Bahram V, the persecution continued, and large numbers of Christians fled across the frontier into Roman territory. A bitter feeling between Persia and Rome grew out of Bahram's demand for the surrender of the Christian fugitives, and war was declared in 422. The conflict commenced with Roman success in Armenia and the capture of a large number of Persian prisoners; the Romans then advanced into Persia and ravaged the border province of Azarena, but the seat of war was soon transferred to Mesopotamia, where the Romans besieged Nisibis. The Persians, hard pressed in this siege, called in the Turks to their assistance, and the united armies marched to the relief of the city. The Romans were alarmed at the news of the large numbers of the Persian forces and raised the siege, but soon afterwards, when the turks had retired, there was a general engagement in which the Romans inflicted a crushing defeat upon their adversaries, and compelled them to sue for peace. Although the latter half of the fourth and the beginning of the fifth century was a period of so much distress in the Eastern provinces, which were exposed to the growing ambition of Persia, it was a time of extension of the Christian Church and of literary activity. This literary and ecclesiastical development led to the formation of a Syriac literature in Persia (Syriac being the liturgical language of the Persian Church), and ultimately of a Christian Persian literature.
Towards the middle of the fifth century, the Persian Emperor Yezdegerd (442-59) was compelled to turn his attention to the passes of the Caucasus; troops of Huns and Scythians had already broken through into Iran. Peroses (Firuz), his successor, made war on the nomads of the Caspian regions, and in 484 lost his life in battle with them. Four years later the throne of Persia was occupied by Qubad I, who reigned from 488 to 531. During this reign there developed in Persia a new sect of the Fire-worshippers (the Mazdakeans), who were at first favoured by the king, but who subsequently involved the empire in serious complications. The last decade of Qubad's reign was chiefly occupied by wars with the Romans, in which he found a good means for diverting the attention of his people from domestic affairs. During the very last days of his life Qubad was compelled once more to lead an army to the West to maintain Persia's influence over Lasistan in southern Caucasia, the prince of which country had become a convert to Christianity, and consequently an ally of the Byzantine empire. It was during the same reign that the Nestorians began to enter more fully into Persian life, and under him that they began their missionary expansion eastwards. About the year 496 the patriarchal See of Seleucia-Ctesiphon fell into the hands of the Nestorians, and henceforth the Catholicos of Seleucia became the patriarch of the Nestorian Church of Persia, Syria, China, and India. After the death of Qubad the usual quarrels as to the succession arose, and finally ended, in 531, with the accession of Chosroes I Anushirwân whom Qubad had looked upon as the most capable of his sons. Chosroes was a champion of the ancient Iranian spirit, a friend of the priest class, and an irreconcilable enemy of the Mazdakites, who had chosen one of his numerous brothers as their candidate for the throne. During his reign the Persian Empire attained the height of its splendour; indeed, the government of Chosroes I, "the Just", was both equitable and vigorous. One of his first acts was to make peace with Byzantium, the latter agreeing pay a large contribution towards the fortification of the Caucasian passes. In addition to strengthening the Caucasus, Chosroes also sought to fortify the north-eastern frontier of his empire by constructing a great wall, and he asserted his claims to a portion of northwestern India by force of arms, but son turned his attention once more to the West. In 531 he proclaimed a general toleration, in which not only Christians, but also Manichæans and Mazdakites, were included.
The period 532-39 was spent in the extension and strengthening of the eastern frontiers of Persia. In 539 Chosroes returned to Ctesiphon, and was persuaded by the Bedouin Al Mondar to renew Qubad's attempted conquest of Syria. The pretext was that Justinian was aiming at universal dominion, but there is no doubt that the real reason was that Al Mondar remembered the ease with which he had once plundered Syrian territory. In 540 the Persians invaded Syria and captured the city of Shurab. the prisoners taken from this city were released at the request of Candidus, bishop of the neighbouring town of Sergiopolis, who undertook to pay a ransom of 200 pounds of gold. Then Chosroes took Mabbogh, which paid a ransom, then Beroea, and finally proceeded against Antioch itself, which was captured after a short resistance. From Antioch Chosroes carried off many works of art and a vast number of captives. On his way homewards he made an attack upon Edessa, a city generally regarded as impregnable, but was taken ill during the siege.
During Chosroes's illness trouble occurred in Persia. He had married a Christian wife, and his son Nushizad was also a Christian. When the king was taken ill at Edessa a report reached Persia that he was dead, and at once Nushizad seized the crown. Very soon the rumour was prove false, but Nushizad was persuaded by persons who appear to have been in the pay of Justinian to endeavour to maintain his position. The action of his son was deeply distressing to Chosroes; but it was necessary to take prompt measures, and the commander, Ram Berzin, was sent against the rebels. In the battle which followed Nushizad was mortally wounded and carried off the field. In his tent he was attended by a Christian bishop, probably Mar Aba, and to this bishop he confessed his sincere repentance for having taken up arms against his father, an act which, he was convinced, could never win the approval of Heaven. Having professed himself a Christian he died, and the rebellion was quickly put down.
Mar Aba was probably the Nestorian Catholicos from 536 to 552. He was a convert from Zoroastrianism, and had studied Greek at Nisibis and Edessa, making use of his knowledge to prepare and publish a new version of the Old Testament. This appears to have been a total failure, for the Nestorians, unlike the Jacobites, steadily adhered to the Peshito. On being appointed catholicos he established a school at Seleucia, which soon became a great centre of Nestorian scholarship. He wrote commentaries, homilies, and letters, the two former classes of work representing, no doubt, the substance of his teaching in the school which he founded. Hymns are extant which are ascribed to him. Chosroes, after his return from Syria, taunted Mar Aba with professing a type of Christianity unknown to the rest of the world. But Mar Aba did much to remove the more marked peculiarities of the Nestorian schism, especially again enforcing celibacy amongst the bishops. From time to time he held discussions with Chosroes, until on one occasion, being tactless enough not to be convinced by the arguments of the sovereign, he was sentenced to banishment. As he disobeyed the decree, he was cast into prison, where he died in 552. In 542 Chosroes claimed from Bishop Candidus the payment of the sum to which he had pledged himself as ransom for the captives taken at Shurab; but the bishop was unable to raise the money; in fact he confessed that he had only made the promise in the expectation that the Government would find part of the sum required, and this had not been done. Therefore Candidus was put to death. In the course of the same year Chosroes advanced south and attacked Jerusalem, but was repulsed by Belisarius.
Mar Aba's foundation of a school at Seleucia seems to have suggested to Chosroes the idea of founding a Zoroastrian school similar to it and to the Christian instructions at Edessa and Nisibis. In pursuance of this plan the king opened a college at Djundi Shapur, and here many Greek, Syrian, and Indian works were translated into Persian, and the ancient laws of Persia were rendered into the vernacular dialect (Pahlavi). Meanwhile the school at Seleucia became a centre of Nestorian life. It was a period during which the Nestorians were returning to a greater conformity to the usages of the rest of Christendom. We have already mentioned Mar Aba's restoration of celibacy, at least as far as the bishops were concerned. About the same time two distinguished monks, both bearing the name of Abraham of Kashkar, introduced reforms into monastic life which also tended towards conformity with the practices of the Church within the Roman Empire. Probably this tendency to conformity was due to increase of Greek influence observable during the reign of Chosroes, and the contact with the empire due to the invasion of Syria; nevertheless the Nestorians remained a distinct body.
Meanwhile the Catholicos Mar Aba had died, and Chosroes appointed his favourite physician, Joseph, as Bishop of Seleucia (552). Many strange stories are related of his cruelty as bishop; after three years he was deposed on a petition of the Christians of Seleucia. He lived twelve years after his deposition, and during that period no catholicos was appointed. About the same time the indefatigable Jacob Burdeana consecrated Achudemma as Jacobite bishop in Persia, and made a proselyte of a member of the royal family. Amongst the Persians it was never permitted to make converts from the state religion. The Jacobites however were of little importance so far east, where Nestorianism was the prevailing type of Christianity. After the death of Joseph in 567. Ezechiel, a disciple of Mar Aba, was appointed Catholicos of Seleucia, under whom lived the periodeutes Bodh, the translator into Syriac of the Indian tales known as "Kalilah and Dimnah". It is noteworthy that the Nestorians were beginning to take an interest in Indian literature, an interest probably to be referred to the influence of the Djundi Shapur school.
Chosroes was succeeded by his son Hormuz (579-90). For the firs three years of his reign Hormuz was guided by the statesman-philosopher Buzurg, but after his retirement Hormuz gave himself up to every form of self-indulgence and tyranny. Under these conditions the power of Persia declined, and the land suffered invasion on the north, east, and west. To check the Byzantines, Bahram, a general who had distinguished himself under chosroes, was sent to invade Colehis, but he was defeated and recalled in disgrace. Knowing that this was equivalent to sentence of death, Bahram revolted, and succeeded in capturing Hormuz, whom he put to death. Chosroes, the king's son, fled and was well received by Probus, Governor of Circesium, and afterwards by the Emperor Mauritius. With the help of the Romans this younger Chosroes defeated Bahram, and became king as Chosroes II. As he owed his kingdom and his wife to the Emperor Mauritius, Chosroes was devoted to the dynasty then reigning at Constantinople. Although not himself a Christian, he paid honour to the Blessed Virgin and to the martyrs Sergius and Bacchus, two saints popular among the Syrians, while his wife as an ardent Jacobite.
In 604 the Roman Emperor Mauritius was assassinated, and the Persian king resolved to attack the empire in order to avenge his benefactor. In 604 the Persians again invaded the eastern provinces and took the city of Daras. The invasion of Chosroes II was the severest blow that the Byzantine power in Asia had to endure, previous to the rise of Islam. After five years of war Chosroes II reached Constantinople. It was not a mere plundering expedition, but a serious invasion whose success clearly proved the growing weakness of the Byzantine Empire. Next year (606) the invaders reached Amida; in 607 they were at Edessa; in 608 at Aleppo; and by 611 they had conquered all northern Syria, and established themselves at Antioch. They then turned south and conquered Palestine. In 615 Jerusalem revolted, but was cruelly punished, some 17,000 persons being put to death, and about 35,000 led away captive. The fragment of the True Cross, the most precious relic of the city, was carried off. Next year (616) the Persians took Alexandria, and in 617 besieged Constantinople. Although the imperial city was not taken, Asia Minor remained in the hands of the Persians until 624.
Chosroes II was repelled, not by the Romans, but by a people who were yearly growing more powerful, and were destined ultimately to displace both Rome and Persian in Asia — the Arabs. Chosroes II had a harem of 3,000 wives, as well as 12,000 female slaves, but he now demanded as wife Hadiqah, the daughter of the Christian Arab Na'aman, himself the son of Al Mondir. Na'aman refused to permit his Christian daughter to enter the harem of a Zoroastrian, and for this refusal he was trampled to death by an elephant, whilst Hadiqah took refuge in a convent. The news of this outrage upon an Arab provoked all the Bedouin tribes, and the Arabs revolted. Chosroes II was totally defeated, and fled to the Emperor Heraclius. This victory made a great impression upon the Arab mind, and probably led to the Mohammedan conquests.
The Mohammedan and modern periods (A.D. 651-1911) During the reign of Yezdegerd III, the successor of Chosroes II, and the last of the Sassanian kings, the Arab invaders attacked Persia and its Mesopotamian territories more and more boldly. In 650 Khâlid, one of the Arab generals, assuming the offensive, defeated the Persian troops on the border of the Euphrates valley. The Christians of this region soon submitted to him. Then the Arabs invaded the country about the Tigris. In 634 Abu Ubaid of Taif, to whom Khâlid assigned the task of annexing Persia, was utterly defeated and slain by the Persians, who, however, were routed in 635-66 by Caliph Omar at Bowaib. Towards the close of the year 636, or in 637, they were again defeated by the Arabs, under Sa'd, at Kadisiyya. The victorious Arabs entered Babylonia and took Seleucia after a lengthy siege. Thence they crossed the Tigris and fell on Ctesiphon, Yezdegerd fleeing towards the Medo-Babylonian frontier. Meantime another army of Arabs had occupied Lower Irâk and entered Susiana. The decisive and final victory took place in 640-42 at Nehavend, near Ecbatana, when the great Persian Empire and the Sassanian dynasty were completely destroyed.