The Roman–Parthian War of 58–63 was fought between the Roman Empire and the Parthian Empire over control of Armenia, a vital buffer state between the two realms. Armenia had been a Roman client state since the days of Emperor Augustus, but in 52/53, the Parthians succeeded in installing their own candidate, Tiridates, on the Armenian throne.
These events coincided with the ascension of Nero to the imperial throne in Rome, and the young emperor decided to react vigorously. The war, which was the only major foreign campaign of his reign, began with rapid success for the Roman forces, led by the able general Gnaeus Domitius Corbulo. They overcame the forces loyal to Tiridates, installed their own candidate, Tigranes VI, on the Armenian throne, and left the country. The Romans were aided by the fact that the Parthian king Vologases was embroiled in the suppression of a series of revolts in his own country. As soon as these had been dealt with, however, the Parthians turned their attention to Armenia, and after a couple of years of inconclusive campaigning, inflicted a heavy defeat on the Romans in the Battle of Rhandeia.
The conflict ended soon after, in an effective stalemate and a formal compromise: a Parthian prince of the Arsacid line would henceforth sit on the Armenian throne, but his nomination had to be approved by the Roman emperor.  This conflict was the first direct confrontation between Parthia and the Romans since Crassus' disastrous expedition and Mark Antony's campaigns a century earlier, and would be the first of a long series of wars between Rome and Iranian powers over Armenia.
Ever since the expanding Roman Republic and the Parthian Empire had come into contact in the mid-1st century BC, there had been friction between the two great powers over the control of the various states lying between them. The largest and most important of these was the Kingdom of Armenia. In 20 BC, Augustus  succeeded in establishing a Roman protectorate  over the country, when Tigranes III was enthroned as king of Armenia. Roman influence was secured through a series of Roman-sponsored kings until 37 AD, when a Parthian-supported candidate, Orodes, assumed the throne. The Roman-supported king, Mithridates, recovered his throne with the support of Emperor Claudius  in 42 AD, but was deposed in 51 AD by his nephew Rhadamistus  of Iberia. His rule quickly became unpopular however, and this gave the newly-crowned king Vologases I of Parthia the opportunity to intervene. His forces quickly seized the two capitals of Armenia, Artaxata and Tigranocerta, and put his younger brother Tiridates on the throne. The onset of a bitter winter and the outbreak of an epidemic forced the Parthians forces to withdraw, allowing Rhadamistus to retake control of the country. His behavior towards his subjects, however, was even worse than before, and they rose in rebellion against him. Thus in 54 AD Rhadamistus fled to his father's court in Iberia, and Tiridates re-established himself in Armenia.
In the same year, in Rome, Emperor Claudius died and was succeeded by his stepson Nero. The Parthian encroachment in an area regarded as lying within the Roman sphere of influence worried the Roman leadership, and was widely seen as a major test of the new emperor's ability. Nero reacted vigorously, appointing Gnaeus Domitius Corbulo, a general who had distinguished himself in Germania and now served as governor of Asia, to supreme command in the East.
Corbulo was given control over two provinces, Cappadocia and Galatia (modern-day central Turkey), with propraetorial and later proconsular  authority or imperium. Although Galatia was considered a good recruiting-ground and Cappadocia had a few units of auxiliaries, the bulk of his army came from Syria, where half the garrison of four legions and several units of auxiliaries was transferred to his command.
Initially, the Romans hoped to resolve the situation by diplomatic means: Corbulo and Ummidius Quadratus, the governor of Syria, both sent embassies to Vologases, proposing that he give up hostages, as was customary during negotiations, to ensure good faith. Vologases, himself preoccupied by the revolt of his son Vardanes which forced him to withdrew his troops from Armenia, readily complied. A period of inactivity ensued, while the Armenian issue remained in limbo. Corbulo used this lull to restore his troops' discipline and combat readiness, which had diminished in the peaceful garrisons of the East. According to Tacitus, Corbulo discharged all who were old or in ill health, kept the entire army under canvas in the harsh winters of the Anatolian plateau to acclimatize them to the snows of Armenia, and enforced a strict discipline, punishing deserters by death. At the same time however, he took care to be constantly present amongst his men, sharing their hardships.
In the meantime, Tiridates, backed by his brother, refused to go to Rome, and even engaged in operations against those Armenians whom he deemed were loyal to Rome. Tension mounted and finally, in the early spring of 58, war broke out.
Corbulo had placed a large number of his auxiliaries in a line of forts near the Armenian frontier under a former primus pilus, Paccius Orfitus. Disobeying Corbulo's orders, he used some newly arrived auxiliary cavalry alae to stage a raid against the Armenians, who appeared to be unprepared. In the event, his raid failed, and the retreating troops even spread their panic amongst the garrisons of the other forts. It was an inauspicious start for a campaign, and Corbulo severely punished the survivors and their commanders.
Having drilled his army for two years Corbulo, despite this misadventure, was ready. He had three legions at his disposal (III Gallica and VI Ferrata from Syria and IV Scythica), to which were added a large number of auxiliaries and allied contingents from Eastern client kings like Aristobulus of Lesser Armenia and Polemon II of Pontus. The situation was furthermore favorable to the Romans: Vologases faced a serious revolt by the Hyrcanians in the region of the Caspian Sea as well as incursions of Dahae and Sacae nomads from Central Asia, and was unable to support his brother.
The war thus far had featured mostly skirmishing along the Roman–Armenian border. Corbulo tried to protect the pro-Roman Armenian settlements from attack, and simultaneously retaliated against the Parthians' supporters. Given that Tiridates avoided confrontation in a pitched battle, Corbulo divided his force, so that they could attack several places simultaneously, and instructed his allies, Kings Antiochus IV of Commagene and Pharasmanes I of Iberia to raid Armenia from their own territories. In addition, an alliance was concluded with the Moschoi, a tribe living in northwestern Armenia.
Tiridates reacted by sending envoys to ask why he was under attack, since hostages had been given. To this, Corbulo reiterated the demand to seek the recognition of his crown from Nero. Eventually, the two sides agreed on a meeting. Tiridates announced that he would bring 1,000 men to the meeting, implying that Corbulo should bring the same number of men "in peaceful fashion, without breastplates and helmets". Tacitus suggests that Tiridates intended to overwhelm the Romans, as the Parthian cavalry would be superior to an equal number of Roman infantry in any case. At any rate, in a show of force Corbulo decided to take with him the larger part of his force, not only IV Ferrata, but also 3,000 men from III Gallica plus the auxiliaries. Tiridates also appeared at the agreed site, but, seeing the Romans in full battle array, and in turn distrusting their intentions, he did not come closer and withdrew during the night. Tiridates then resorted to a tactic that had worked well a century earlier against Mark Antony: he sent forces to raid the Roman army's supply route, which stretched over the mountains back to Trapezus in the Black Sea. They failed however, since the Romans had taken care to secure the mountain routes by a series of forts.
Corbulo now resolved to directly attack Tiridates' fortified strongholds. Not only were they instrumental in controlling the surrounding country and sources of revenue and soldiers, but in addition, a threat to them might force Tiridates to risk a pitched battle, since, in the words of historian A. Goldsworthy, "a king who could not defend communities loyal to him [...] lost prestige."  Corbulo and his subordinates successfully stormed three of these forts, including Volandum (possibly modern Iğdır),  "the strongest of all in that province" according to Tacitus, within a day with minimal casualties, and massacred their garrisons. Terrified by this display of Roman might, several towns and villages surrendered, and the Romans prepared to move against the northern Armenian capital, Artaxata.
This forced Tiridates to confront the Romans with his army, as they approached Artaxata. The Roman force, reinforced by a vexillatio of X Fretensis, marched in a hollow square, with the legions supported by auxiliary horsemen and foot archers. The Roman soldiers were under strict orders not to break formation, and despite repeated probing attacks and feigned retreats by the Parthian horse archers, they held together until nightfall. During the night, Tiridates withdrew his army, abandoning his capital, its inhabitants promptly surrendered and were allowed to leave unmolested, but the city was torched, since the Romans could not spare sufficient men for garrisoning it.
In 59, the Romans marched to the south, towards Tigranocerta, Armenia's second capital city. On their way, Corbulo's men punished those who withstood or hid from them, while leniency was shown to those who surrendered.  In the harsh, dry terrain of northern Mesopotamia, the army suffered from lack of provisions, especially water, until they reached the more fertile areas near Tigranocerta. During that time, a plot to murder Corbulo was uncovered and suppressed. Several Armenian nobles who had joined the Roman camp were implicated and executed.  According to a story provided by Frontinus, when the Roman army arrived at Tigranocerta, they launched the severed head of one of the conspirators into the city. By chance, it landed right where the city council was assembled, they immediately decided to surrender the city, which was consequently spared.  Shortly after, an attempt by the Parthian army under king Vologases to enter Armenia was blocked by Verulanus Severus, the commander of the auxiliaries.
The Romans were now in control of Armenia, and they promptly installed its new king, Tigranes VI, the last descendant of the Cappadocian royal house, in Tigranocerta. Some outlying western parts of Armenia were also ceded to the Roman vassals. Corbulo left 1,000 legionaries, three auxiliary cohorts and two cavalry alae (ca. 3-4,000 men) behind to support the new monarch, and retired with the rest of his army to Syria, whose governorship he now (in 60 AD) assumed as a reward for his success.
The Romans were well aware that their victory was still fragile, and that as soon as the Parthian king had dealt with the Hyrcanian rebellion, he would turn his attention to Armenia. Despite Vologases' reluctance to risk an all-out conflict with Rome, in the end, he was forced to act when Tigranes raided the Parthian province of Adiabene  in 61. The enraged protests of its governor Monobazus, and his pleas for protection, could not be ignored by Vologases, whose prestige and royal authority were at stake.  Vologases therefore hastily concluded a treaty with the Hyrcanians so as to be free to campaign against Rome, and called an assembly of the grandees of his realm. There he publicly reaffirmed Tiridates' position as king of Armenia by crowning him with a diadem. In order to reinstall his brother on the Armenian throne, the Parthian king assembled a force of picked cavalry under Monaeses, complemented by infantry from Adiabene.
In response, Corbulo sent the legions IV Scythica and XII Fulminata to Armenia, while he detailed the three other legions under his command (III Gallica, VI Ferrata and XV Apollinaris) to fortify the line of the river Euphrates, fearing that the Parthians might invade Syria. At the same time, he petitioned Nero to appoint a separate legate for Cappadocia, with the responsibility for conducting the war in Armenia.
Monaeses meanwhile entered Armenia and approached Tigranocerta. Tigranes had taken care to gather supplies, and the city was well-fortified and garrisoned with Romans and Armenians alike. The siege was largely undertaken by the Adiabenian  contingent, since the Parthians, being cavalrymen, were unskilled and unwilling to engage in siegework.  The Parthian assault failed, and was driven back with loss by a successful Roman sally.  At this point, Corbulo sent an envoy to Vologases, who had encamped with his court at Nisibis, near Tigranocerta and the Roman–Parthian border. The failed siege and a shortage of fodder for his cavalry forced Vologases to agree to withdraw Monaeses from Armenia.  At the same time however, the Romans too left Armenia, something that, according to Tacitus, raised suspicions of Corbulo's motives: some whispered that he had reached an agreement of mutual withdrawal with the Parthians, and that he was unwilling to risk his reputation by renewing hostilities against them.  At any rate, a truce was arranged and a Parthian embassy was dispatched to Rome. The negotiations failed to reach an agreement, and war was resumed in the spring of 62.
In the meantime, the asked-for legate for Cappadocia had arrived, in the person of Lucius Caesennius Paetus, the consul of the previous year (61 AD). The army was divided between him and Corbulo, with IV Scythica, XII Fulminata, the newly arrived V Macedonica and the auxiliaries from Pontus, Galatia and Cappadocia going to Paetus, while Corbulo retained III Gallica, VI Ferrata and X Fretensis. Because of their antagonism for glory, the relations between the two Roman commanders were strained from the beginning. It is notable that Corbulo kept the legions he had spent the past few years campaigning with, and gave his colleague, who after all was expected to conduct the main campaign, the more inexperienced units. The total Roman force arrayed against the Parthians was nevertheless considerable: the six legions alone totaled some 30,000 men. The exact number and disposition of auxiliary units is unclear, but there were seven cavalry alae and seven infantry cohorts in Syria alone, comprising a force of 7-9,000 troops.
Paetus nonetheless appeared confident of victory, and followed the Parthian declaration of war and capture of Tigranocerta with his own invasion of Armenia,  while Corbulo remained at Syria, further strengthening the fortifications in the Euphrates frontier.  Paetus had only two legions with him, IV Scythica and XII Fulminata,  and advanced towards Tigranocerta. A few minor forts were taken, but a lack of supplies forced him to withdraw westwards for the winter.
The Parthians had originally intended to invade Syria, but Corbulo put on a convincing display of military might, building a strong flotilla of ships equipped with catapults and a bridge over the Euphrates, which allowed him to establish a foothold on the Parthian shore. Therefore the Parthians abandoned their plans for Syria, and turned their attention towards Armenia. There, Paetus had dispersed his forces and granted extended leaves to his officers, so that he was taken unawares at the Parthian advance. Upon learning of it, he initially advanced to meet Vologases, but after a reconnaissance detachment was defeated, he panicked and withdrew hastily. Paetus sent his wife and son to safety in the fortress of Arsamosata, and tried to block the Parthian advance by occupying the passes of the Taurus mountains with detachments from his army. In so doing, however, he further dispersed his forces, which were then defeated in detail by the Parthians. Roman morale plunged and panic set in among the army, which was now besieged in a series of hastily erected camps near Rhandeia. Paetus, who appears to have fallen into desperate inactivity, sent urgent messages to Corbulo to come to his rescue.
Corbulo in the meantime had been aware of the danger faced by his colleague, and put part of his forces on readiness, but he did not march to join Paetus, and some accused him of delaying in order to reap more glory from rescuing him.  Nevertheless, when the calls for assistance arrived, he responded quickly and marched forth with half of the Syrian army, carrying many provisions laden on camels. He soon met dispersed men of Paetus' army, and managed to rally them around his force.  But before he could arrive to the rescue, Paetus had capitulated: the Parthians, aware that relief was nearing, increasingly harassed the Romans, until Paetus was forced to send a letter to Vologases to seek terms.  The subsequent treaty was humiliating: not only would the Romans leave Armenia and surrender all forts they held, but they also agreed to build a bridge over the nearby Arsanias river over which Vologases could pass in triumph, sitting atop an elephant.  In addition, the Roman army was liberally plundered by the Armenians, who took even the Romans' weapons and clothes without facing any resistance. Still worse, according to rumors reported by Tacitus, the Romans were made to pass under the yoke, a gesture of ultimate humiliation in Roman eyes.
The two Roman forces met on the banks of the Euphrates near Melitene, amidst scenes of mutual grief, while Corbulo lamented the undoing of his achievements, Paetus tried to convince him to attempt to reverse the situation by invading Armenia. Corbulo however refused, claiming that he did not have the authority to do so, and that either way the army was too worn out to be able to campaign effectively. In the end, Paetus retired to Cappadocia and Corbulo to Syria, where he received envoys from Vologases, who demanded that he evacuate his bridgehead over the Euphrates. In turn, Corbulo demanded the Parthian evacuation of Armenia. Vologases agreed to this, and both sides withdrew their forces, leaving Armenia once again masterless but de facto under Parthian control, until a Parthian delegation could travel to Rome.
Corbulo in the meantime had been aware of the danger faced by his colleague, and put part of his forces on readiness, but he did not march to join Paetus, and some accused him of delaying in order to reap more glory from rescuing him.  Nevertheless, when the calls for assistance arrived, he responded quickly and marched forth with half of the Syrian army, carrying many provisions laden on camels. He soon met dispersed men of Paetus' army, and managed to rally them around his force.  But before he could arrive to the rescue, Paetus had capitulated: the Parthians, aware that relief was nearing, increasingly harassed the Romans, until Paetus was forced to send a letter to Vologases to seek terms.  The subsequent treaty was humiliating: not only would the Romans leave Armenia and surrender all forts they held, but they also agreed to build a bridge over the nearby Arsanias river over which Vologases could pass in triumph, sitting atop an elephant.  In addition, the Roman army was liberally plundered by the Armenians, who took even the Romans' weapons and clothes without facing any resistance. Still worse, according to rumors reported by Tacitus, the Romans were made to pass under the yoke, a gesture of ultimate humiliation in Roman eyes.
The two Roman forces met on the banks of the Euphrates near Melitene, amidst scenes of mutual grief, while Corbulo lamented the undoing of his achievements, Paetus tried to convince him to attempt to reverse the situation by invading Armenia. Corbulo however refused, claiming that he did not have the authority to do so, and that either way the army was too worn out to be able to campaign effectively. In the end, Paetus retired to Cappadocia and Corbulo to Syria, where he received envoys from Vologases, who demanded that he evacuate his bridgehead over the Euphrates. In turn, Corbulo demanded the Parthian evacuation of Armenia. Vologases agreed to this, and both sides withdrew their forces, leaving Armenia once again masterless but de facto under Parthian control, until a Parthian delegation could travel to Rome.
In 66, Tiridates visited Rome to receive his crown and was lavishly received by Nero, who used the occasion to boost his own popularity. He ordered the gates of the Temple of Janus to be shut, thus declaring that peace reigned throughout the Roman Empire.
Nero celebrated this peace as a major achievement: he was hailed as imperator and held a triumph, although no new territory had been won, and the peace reflected a compromise rather than a true victory. For although Rome could prevail militarily in Armenia, politically, she had no genuine alternatives to the Arsacid candidacy on offer for the Armenian throne. Armenia would henceforth be ruled by an Iranian dynasty, and despite its nominal allegiance to Rome, it would come under increasing Parthian influence. In the judgment of later generations, "Nero had lost Armenia", and although the Peace of Rhandeia ushered in a period of relatively peaceful relations that would last for 50 years, Armenia would continue to be a constant bone of contention between the Romans, the Parthians, and their Sassanid successors. For the short term however, the peace that Nero secured was kept by both sides, even while the bulk of Rome's eastern forces was involved in the suppression of the Jewish Revolt.
As for Corbulo, he was feted and honoured by Nero as the man who had brought this "triumph" to be, but his popularity and influence with the army made him a potential rival. Together with the involvement of his son-in-law Lucius Annius Vinicianus in a foiled plot against Nero in 66, Corbulo became suspect in the eyes of the emperor. In 67, while journeying in Greece, Nero ordered him to be executed, upon hearing of this, Corbulo committed suicide.
This inconclusive war had also demonstrated to the Romans that the defensive system in the East, as put in place by Augustus, was no longer adequate. Thus the following years saw a major reorganization of the Roman East: the client kingdoms of Pontus and Colchis (in 64 AD), Cilicia, Commagene and Lesser Armenia (in 72 AD) were made into Roman provinces, the number of legions in the area increased, and Roman presence in the Caucasian client states of Iberia and Albania strengthened, with the aim of strategically encircling Armenia. Direct Roman control was extended to the entire line of the Euphrates, marking the beginning of the Eastern limes that would survive until the Muslim conquests of the 7th centu
