From Yazdegerd I. to the Turks.—In the interior of the Sassanian empire the old troubles broke out anew on the death of Shapur II. (379). At first the magnates raised his aged brother Ardashir II. to the throne, then in 383 deposed him and en throned Shapur's son as Shapur III. In 388, however, he was assassinated, as was also his brother, Bahram IV., in 399. But the son of the latter, Yazdegerd I. was an energetic and intelligent sovereign, who held the magnates within bounds and severely chastised their attempts at encroachment. He even sought to emancipate himself from the Magian Church, put an end to the persecutions, and allowed the Persian Christians an individual organization. In the Persian tradition he is conse quently known as "the sinner." In the end he was probably assassinated. So great was the bitterness against him that the magnates would admit none of his sons to the throne. One of them, however, Bahram V., found an auxiliary in the Arab chief Mondhir, who had founded a principality in Hira, west of the lower Euphrates; and, as he pledged himself to govern otherwise than his father, he received general recognition. This pledge he redeemed, and he is, in consequence, the darling of Persian tra dition, which bestows on him the title of Gor ("the wild ass"), and is eloquent on his adventures in the chase and in love. This reversal of policy led to a Christian persecution and a new war with Rome. Bahram, however, was worsted; and in the peace of 422 Persia agreed to allow the Christians free exercise of their religion in the empire, while the same privilege was accorded to Zoroastrianism by Rome. Under his son, Yazdegerd II. (438 457), who once more revived the persecutions of the Christians and the Jews, a short conflict with Rome again ensued (441) : while at the same time war prevailed in the east against the remnants of the Kushan empire and the tribe of Kidarites, also named Huns.
Here a new foe soon arose in the shape of the Ephthalites (Haitab), also known as the "White Huns," a barbaric tribe which shortly after A.D. 450 raided Bactria and terminated the Kushana dominion (Procop. Pers. i. 3), and soon began to ex tend their invasions into India, where they destroyed the Gupta empire about A.D. 500. These Ephthalite attacks harassed and weakened the Sassanids, exactly as the Tocharians had harassed and weakened the Arsacids after Phraates II. Peroz fell in battle against them ; his treasures and family were cap tured and the country devastated far and near. His brother Balash being unable to repel them, was deposed and blinded, and the crown was bestowed on Kavadh I. the son of Peroz. As the external and internal distress still con tinued he was dethroned and imprisoned, but took refuge among the Ephthalites aM was restored in 499 by their assistance—like so many Arsacids by the arms of the Dahae and Sacae. To these struggles obviously must be attributed mainly the fact that in the whole of this period no Roman war broke out. But, at the same time, the religious duel had lost in intensity, since, among the Persian Christians, the Nestorian doctrine was now dominant. Peroz had already favoured the diffusion of Nestorianism, and in 483 it was officially adopted by a synod, after which it re mained the Christian Church of the Persian empire, its head being the patriarch of Seleucia-Ctesiphon.
Kavadh proved himself a vigorous ruler. On his return he restored order in the interior. In 502 he attacked the Romans and captured and destroyed Amida (mod. Diarbekr), but was com pelled to ratify a peace owing to an inroad of the Huns. Toward the close of his reign (527) he resumed the war, defeating Beli sarius at Callinicum (531), with the zealous support of the wild Arab Mondhir II. of Hira. On his death his son Chosroes I. concluded a peace with Justinian (532), pledging the Romans to an annual subsidy for the maintenance of the Caucasus f or tresses. In his home policy Kavadh is reminiscent of Yazdegerd I. Like him he had little inclination to the orthodox church, and favoured Mazdak, the founder of a communistic sect which had made headway among the people and might be used as a weapon against the nobles, of whom Mazdak demanded that they should cut down their luxury and distribute their superfluous wealth. Another feature of his programme was the community of wives. The crown-prince, Chosroes, was, on the other hand, wholly orthodox; and, towards the close of his father's reign, in con junction with the .chief Magian, he carried through the con demnation of the Mazdakites, who were butchered in a great massacre (528). Chosroes I. (531-579), surnamed Anushirvan ("the blessed"), then restored the orthodox doctrine in full, pub lishing his decision in a religious edict. At the same time he pro duced the official exposition of the Avesta, an exegetical trans lation in the popular tongue (Pahlavi), and declared its contents binding. Defection from Zoroastrianism was punished with death, and therefore also the proselytizing of the Christians, though the Syrian martyrologies prove that the kings frequently ignored these proceedings so long as it was at all possible to do so.
Chosroes I. was one of the most illustrious sovereigns of the Sassanian empire. From him dates a new and equitable adjust ment of the imperial taxation, which was later adopted by the Arabs. His reputation as an enlightened ruler stood so high that when Justinian, in 529, closed the school of Athens, the last Neo platonists bent their steps to him in hopes of finding in him the true philosopher-king. Their disillusionment, indeed, was speedy and complete, and their gratitude was great, when, by the con ditions of the armistice of 549, he allowed their return. From 54o onward he conducted a great war against Justinian (527-565), which, though interrupted by several armistices, lasted till the 5o years' peace of 562. The net result, indeed, was merely to restore the status quo; but during the campaign Chosroes sacked Antioch and transplanted the population to a new quarter of Ctesiphon (54o). He also extended his power to the Black sea and the Caucasus ; on the other hand, a siege of Edessa failed (544)• A second war broke out in 577, chiefly on the question of Armenia and the Caucasus territory. In this Chosroes ravaged Cappadocia in 575; but the campaign in Mesopotamia was un successful. In the interval between these two struggles (57o) he despatched assistance to the Arabs of Yemen, who had been assailed and subdued by the Abyssinian Christians ; after which period Yemen remained nominally under Persian suzerainty till its fate was sealed by the conquests of Mohammed and Islam.