Persia was a rival of the Roman Empire since the third century BC. When Roman Emperor Constantine converted to Christianity in 312 AD, the Persians started regarding Christianity as a Roman religion. While Constantine had not made Christianity a state religion, his personal ambition was to be the protector of all Christians, including those who were living outside the borders of the Roman Empire. He adopted a strident attitude towards Persia, which was then being ruled by the Sasanian dynasty (224-651 AD) who were Zoroastrian. Zoroastrianism was deeply rooted in Persia, having arrived in the country in the second millennium BC.
In the final years of his life, Constantine began to contemplate a military campaign against Persia. In a letter to Persian Emperor Shapur II, Constantine declared that he was delighted to know that Persia was home to a significant number of Christians whose faith was like his own and that he would advise Shapur II to treat his Christian subjects well. Constantine’s letter sounded like a threat. He was eliding the promotion of Rome’s geopolitical interests with that of his new Christian faith. Shapur II was incensed.
There was no cause for Constantine to believe that the Christians were being mistreated in Persia. While Zoroastrianism was the state religion of Persia, other religions were not facing persecution in the country. Moreover, there were too few Christians in Persia in that time for the Persian regime to take note of their existence. The Romans and the Persians had been fighting wars for more than six centuries. The Romans had contempt for Zoroastrianism, and the Persians were contemptuous of Roman paganism. But their wars were over geopolitical issues and not religious differences. In 297 AD, Narseh of Persia and Diocletian of Rome had signed a treaty (Peace of Nisibis) which led to a period of peace between the two empires.
Before he could march his troops into Persia, Constantine fell ill and died in 337 AD. But in his letter to Shapur II, he had already created a convincing casus belli against the Persians. Most historians agree that it is Shapur II who broke the peace treaty of 297 AD by moving into Roman Mesopotamia. The two empires became embroiled in long drawn wars (337–350 AD and 358-363 AD). The Persians blamed Constantine’s conversion to Christianity for the conflict, and they started viewing the Christians living in their land as the secret supporters of Rome. Shapur II and his successors launched a series of persecutions which made martyrs of several Christians.
Blindsided by their obsession with destroying each other, the Romans and the Persians failed to take timely action to curb the barbarian tribes: Goths, Alans, Huns, Vandals, Suebi, and the nomadic Turks. By the fifth century, some of these tribes had become a serious threat to Rome and Persia. Rome was conquered by the Visigoths in the fifth century. Persia was destabilized and economically ruined by the barbarians but they survived till the seventh century, when a new force arose: Islam. The Sasanian dynasty was toppled by the groups fighting under the banner of Islam. Zoroastrianism was overthrown. By the eighth century, Islam had conquered almost the entire Levant, and significant parts of North Africa, East Asia, and Southwestern Europe.
Constantine’s conversion was good for Christianity in the West, but it proved to be a disaster for Christianity and Zoroastrianism in the East. According to a 2011 census, there are 25,000 Zoroastrians living in modern day Persia (Iran)—in Constantine’s time, ninety-seven percent of Persia was Zoroastrian.