His reign was marked by wars against the Eastern Roman Empire in the west and the Kidarites in the east, as well as by his efforts and attempts to strengthen royal centralisation in the bureaucracy by imposing Zoroastrianism on the non-Zoroastrians within the country, namely the Christians.

This backfired in Armenia, culminating in a large-scale rebellion led by the military leader Vardan Mamikonian, who was ultimately defeated and killed at the Battle of Avarayr in 451.

[5] Since the reign of Shapur II (r. 309–379), Iran had to deal with nomadic invaders in the east known as "Iranian Huns" and made up of Hephthalites, Kidarites, Chionites and Alkhans).

[10] Hard-pressed by the Huns, Iran fought an almost uninterrupted war with them on its northern and northeastern marches, notably under Bahram V and Yazdegerd II, who both attempted to regain Tokharistan, but only succeeded in preserving Abarshahr.

[14] In 450, he launched an expedition into deep Kidarite territory in Central Asia, raiding and capturing forts and cities, which resulted in the accumulation of many captives and riches.

[15] In 453, he moved his court to Nishapur in Abarshahr to face the threat from the Kidarites and left his minister (wuzurg framadar) Mihr Narseh in charge of the Sasanian Empire.

[15] According to the Šahrestānīhā ī Ērānšahr ("The Provincial Capitals of Iran"), Yazdegerd II fortified the city of Damghan and turned it into a strong border post against the Kidarites.

[17] After he managed to secure the eastern portion of his empire against the Kidarite incursions, Yazdegerd II shifted his focus on Armenia and Caucasian Albania to defend the Caucasus with the Romans against the increasing Hun threat.

[19] At the beginning of Yazdegerd II's reign, he suffered several defeats at the hands of the Kidarites, for which he put the blame on the Christians, due to much of his cavalry consisting of Iberians and Armenians.

When the magnates told him that his new policies had offended the people, he disagreed, saying that: "it is not correct for you to presume that the ways in which my father behaved towards you, maintaining you close to him, and bestowing upon you all that bounty, are incumbent upon all the kings that come after him ... each age has its own customs".

[20] Yazdegerd II, however, was still fully aware of the longstanding conflict between the crown and the nobility and priesthood, which had culminated in the murder of several Sasanian monarchs.

[21] Yazdegerd II's primary goal throughout his reign was thus to combat the internal and external issues posing a danger to country by strengthening the royal centralisation of the bureaucracy, which demanded the cooperation of the aristocracy.

[23] He also targeted Zoroastrian aristocrats, dismantling their advantage of entry to the court and castrated men in his field armies to generate eunuchs more dutiful to him than to their own families.

[24] However, Yazdegerd II's policy of integrating the Christian nobility into the bureaucracy still had problematic consequences; before the appointment of Adhur-Hormizd, Armenia had been plunged into a major rebellion.

[27] Jews were also the subject of persecution under Yazdegerd II; he is said to have issued decrees prohibiting them from observing the Sabbath openly,[28] and ordered executions of several Jewish leaders.

[5] This resulted in the Jewish community of Spahan publicly retaliating by flaying two Zoroastrian priests alive, leading in turn to more persecutions against the Jews.

[34] Near the city, he founded the fortified settlement of Shahristan-i Yazdegerd (present-day ruins of Torpakh-kala), which became the main hub of the soldiers stationed in the region, whose leader held the title of "marzban of Chol".

[39] The war against the Hunnic tribes may have awakened the mythical rivalry existing between the Iranian Kayanian rulers and their Turanian enemies, which is demonstrated in the Younger Avesta.