Yazdegerd has tried to emancipate himself from the dominion of the magnates and of the Magi (a priestly caste serving a number of religions); thus, Christian and Zoroastrian sources view his reign differently.

Because he had stopped the persecution of the Christians, the Christian writers praise his clemency, but the sources dependent on Zoroastrian tradition refer to him as “Yazdegerd the Sinner.” He has also tried to limit the power of the nobles, but had finally answered their resistance with severity.

He has lived in peace and friendship with the Roman Empire and is therefore praised by Greek authors.

He appears to have been murdered in Khorasan, after which the nobles refuse to admit any of his sons to the throne.

However, one of them, Bahram, has the support of al-Mundhir, Lakhmid Arab king of Al-Hirah in Mesene (east of the lower Euphrates) and a Sassanian vassal, and also, apparently, of Mihr-Narseh, chief minister in Yazdegerd's last years, who is retained in office.

After Yazdegerd's death in 421, Persian nobles try to reclaim Bahram from Mundhir, so Mundhir sends his son Nu'man ahead with a brigade, then personally escorts Bahram with another brigade of 20,000 soldiers to Ctesiphon, where the nobles, after some negotiations, acknowledge Bahram as their ruler.

Bahram V, known as Gor (“the wild ass,” for his prowess in hunting the onager, reverses his father‘s policy of religious toleration, and revives the violent persecution of Christians in the Persian empire.

War breaks out between Constantinople and Ctesiphon because of the refusal of Theodosius to return to the Persians the Christians who have sought refuge from persecution in his empire.

In addition, the Persians have proven unwilling to return to the Romans gold-miners who had been working in Persian territory; they have also confiscated goods from Roman merchants.

Ardaburius, the emperor's magister militum praesentalis, conducts an invasion of Arzanene by way of Armenia.

After defeating the Persian commander Mihr-Narseh, …

…Ardaburius foils his opponent's attempt to ravage Mesopotamia, and besieges the Persians at Nisibis.

Bahram, in alliance with the Lakhmid Arabs of Hirah, marches with al-Mundhir to lift the siege.

The Roman-Sassanid War escalates as Theodosius transfers further troops to the East, while Bahram personally takes the field, supported by a large contingent of Lakhmid Arabs, and drives the Romans from Nisibis.

Mundhir marches into Roman territory and ravages imperial lands as far as Antioch, where he is routed by Vitianus.

While Al-Mundhir’s troops are crossing the Euphrates on their return, many of his forces drown.

Syriac sources give a figure of those who drowned at seventy thousand while Socrates give a higher figure of one hundred thousand.

Bahram agrees to tolerate Christianity throughout the Persian empire under the one hundred-year peace concluded in 422 between Ctesiphon and Constantinople; the Romans reciprocate by agreeing to tolerate Zoroastrianism throughout the Roman empire.