The Romans made Bazanush, a scion of the royal family, the new Caesar. Realizing the futility of fighting Shahpur, he had a letter written to the Iranian king in which he offered to accept his sovereignty and requested him to stop the war. Shahpur forgave the Romans and agreed to stop the war.

Bazanush went with gifts to thank Shahpur. He was asked to compensate for the destruction he had caused. He asked for a tribute of two hundred thousand Roman dinars thrice a year. He also asked for Nisibis, a Southern province of Kurdistan. Bazanush grudgingly agreed as he knew he would not be able to face the might of the Iranians. In return he asked Shahpur to sign a peace treaty with Rome, to which he agreed.  Thereafter Shahpur returned to Istakhra, the capital of Fars.

When the people of Nisibis, who had been converted to Christianity, came to know of the treaty, they were unhappy. They wanted a Christian and not an Iranian ruler. Shahpur sent an army to Nisibis and crushed their revolt. Only after they asked for forgiveness, did Shahpur call off his army.

Victory at Amida

City and fortress at Amida

In the later years of Shahpur’s reign, there were many wars against the Romans. One of the greatest victory of Sasanian army was at the city of Amida in Mesopotamia on the bank of Tigris (now in eastern Turkey) in 259, which was pro-Roman. Shahpur not only led the siege, but fought shoulder to shoulder with his troops. The army hurled powerful projectiles and battery rams to break the city walls, and finally entered the city after seventy three days. After a long and bloody battle Amida surrendered giving a resounding victory to Shahpur. Soon after that, Singara and Bezabde too were captured.

Renewed Roman attack

In 362, Roman Caesar Julian attacked Persia. He was wishing to defeat Shahpur and have his brother Hormazd, who had defected to Rome, sit on the Iranian throne as a puppet ruler. Julian entered through Mesopotamia, captured Babylon and Seleucia and stood at the gates of the capital city of Ctesiphon. The Roman and Iranian armies met at the bank of the Euphrates. However, the Romans were not able to pierce the strong Iranian defense. Julian, while retreating, was pierced by a javelin and killed on June 26, 363, at the age of 31.

Emperor Jovian assumed command and immediately attacked the Iranians in 363. However, he was not able to defend himself, against the Iranian army, and soon negotiated for peace. Shahpur granted it at a very heavy price. Many major regions including Armenia, Mesopotamia, Singara, Nisibis, and several fortresses came back in Sasanian hands. The Jovian treaty lasted for thirty years. The subsequent Roman emperor Valens attacked the Iranians, but he too sued for peace, on account of which the two kings divided Armenia between themselves in 377.

Religious stability

Shahpur II had to be very firm against Christian infiltrators who had made inroads into Iran on account of the weakness of several previous Iranian rulers. Shahpur came out firmly against them, and curtailed the activities of several over-zealous Christians. He was enraged at the arrogance and defiance of the Christian bishops and so he ordered the closing down of several Churches. At that time, the Roman emperor Constantine II had recently converted himself to Christianity and so he wrote to Shahpur not to be too harsh towards the Christians.

The long reign and firm hands of Shahpur II enabled a lot of religious stability. The Manichaean movement which had taken a very strong foothold among the Iranian nobility and masses especially during the reign of Shapur I and subsequent kings, was put down very firmly by Shahpur II.

Shahpur had many Atash Behrams built during his long reign. Many of his coins had the image of a fire altar on the reverse.

Adarbad Mahraspand

The best thing to happen to Iran during the reign of Shapur II was the rise of a very learned and pious Dastur by the name Adarbad Mahraspand, who was the head priest and was responsible for bringing back faith of the people in religion and rituals after undergoing an ordeal. He had molten brass poured over his chest and came out unscathed from it. In this way he had replied to the challenge to Zoroastrianism from the Zurvanites and the newly founded Christian religion.  After his ordeal he was regarded as the final word on Zoroastrian orthodoxy.

He was the most notable and prolific of the Pahlavi writers in the andarz “admonitions” style of writing. The maximum number of andarz texts are ascribed to him. His writings and sayings are scattered throughout the Pahlavi literature.

The work of collecting and collating the 21 Avestan Nasks was completed under him. He also composed several Pazand prayers like the 6 Setayāsh, Patet Pashemāni and the Āfrins. The Khordeh Avesta, the daily prayer book of the Zoroastrians, was compiled and collated by him. In course of time he came to be regarded as hu-fravart “a saint”.

Shahpur’s reign was the longest among all Sasanian kings. He minted many gold and copper coins. When he was seventy, he asked his younger brother, Ardeshir II, to occupy the throne till his young son, who would become Shahpur III, grew old enough to become the king. Ardeshir, in the presence of ministers, assured his brother Shahpur that he would hand over the crown, throne, treasures and army to prince Shahpur when he would come of age.