Roman – Sasanian War ( 421 – 422 ) The Roman – Sassanid war of 421 – 422 was a conflict between the Eastern Roman Empire and the Sassanids . The " casus belli " was the persecution of Christians by the Sassanid king Bahram V , which had come as a response to attacks by Christians against Zoroastrian temples ; the Christian Eastern Roman Emperor Theodosius II declared war and obtained some victories , but in the end the two powers agreed to sign a peace on the " status quo ante " . Background . In 421 , Bahram V succeeded his father Yazdegerd I , who shortly before he had been killed , began a persecution of Christians as reprisal for attacks against Zoroastrian temples by Christians during his reign ; Bahram continued this persecution , during which many died . Among them there was James Intercisus , a political counsellor of Yazdegerd ' s , who had converted to Zoroastrianism but then converted back to Christianity . The persecuted Christians fled to Roman territory , and were welcomed by the bishop of Constantinople , Atticus , who informed the Emperor of the persecution . The Eastern Roman Emperor Theodosius II was at the time deeply influenced by his religious sister Pulcheria , and had become more and more interested in Christianity . The Roman - Sassanid relationship already had some friction . The Persians had hired some Roman gold - diggers , but now refused to send them back ; furthermore , the Sassanids seized the properties of Roman merchants . So , when Persian ambassadors reached the Roman court to ask for the fugitives , Theodosius choose to break the peace and declare war , rather than giving them back . Conflict . The commander - in - chief of the Roman army was Ardaburius , who , incidentally , came from the Iranian tribe of the Alans . Ardaburius needed to collect many troops for his campaign . Theodosius , therefore , allowed some Pannonian Ostrogoths to settle in Thracia , to defend the province from the Huns while the Thracian Roman troops were sent to the East . Ardaburius sent Anatolius to Persarmenia , where he joined the rebels , while Ardaburius entered in Persian territory and devastated Arzanene . The general of the Sassanid army , Narses , moved with his troops against Ardaburius , and engaged the Romans in battle , but was defeated and forced to retreat . Narses planned to attack Mesopotamia , a Roman province that had been left unguarded , and moved there , but Ardaburius foresaw his enemy ' s plan and reached Mesopotamia . Ardaburius received reinforcements and put the fortress of Nisibis under siege . Bahram allied with the Lakhmid Arabs of Alamundarus ( Al - Mundhir I of Hirah ) , who , however , were dispersed by the Romans . In the meantime , the King of the Huns , Rua , had attacked the dioceses of Dacia and Thracia and had even menaced Constantinople ; at the same time , a large Persian army moved towards Nisibis . To avoid a war on two fronts , Theodosius then recalled Ardaburius back . Siege of Theodosiopolis . According to a Roman ecclesiastical source , the Sassanids besieged Theodosiopolis for 30 days , with thousands of soldiers and even siege engines ( that the source calls " helepolis " ) . According to this source , the Romans did not try to aid the besieged , but the Sassanid were convinced to lift the siege when the bishop of the city , Eunomius , had a stone - thrower , named after Thomas the Apostle , kill a lesser king of the Sassanid army . Despite the evident religious theme of this account , the passage is important as it testifies an unsuccessful Sassanid attack on Theodosiopolis . This could be the Theodosiopolis in Armenia , and in this case the siege should be dated in 421 , while Narses was in Mesopotamia , or Theodosiopolis in Osroene , and in this case the attack should be dated after the Roman retreat from Nisibis . Peace treaty . The peace treaty that ended the war ( 422 ) was negotiated by the " magister officiorum " Helio . It returned everything to the situation before the war ( " status quo ante bellum " ) . Both parts agreed to reject Arab defectors of the other part , as well as to guarantee liberty of religion in their territories . The Romans , however , were obliged to give an annual tribute to the Huns . The most complete account of the war is preserved in Socrates Scholasticus , " Historia Ecclesiastica " VII . 18 , but some passages are included by Theodoret in his " Historia Ecclesiastica " . English translations of these sections are present in :