Parthian–Roman Wars: From Crassus to Septimius Severus

The longest contest in the ancient Near East was fought, most of the time, over a single piece of ground. Armenia — the mountainous country south of the Caucasus, divided between the Roman sphere in the west and the Parthian sphere in the east, ruled for most of the period by a junior branch of the Arsacid house — was the third corner of the triangle that the two empires, and Armenia, kept redrawing for nearly three centuries. The Roman–Parthian wars were, in substance, a series of Armenian wars with a Mesopotamian epilogue. The great invasions — Crassus in 53 BCE, Trajan in 114–117 CE, Lucius Verus in 161–166 CE, Septimius Severus in 194–202 CE — were each triggered, or justified, by an Armenian succession crisis. The Roman–Parthian wars without an Armenian trigger are the exceptions, and they tend to be the smaller engagements.

This article is organised by the major wars, with attention to the Armenian question and to the patterns of Roman-Parthian diplomacy. Touraj Daryaee’s Sasanian Persia (2009) and M. Rahim Shayegan’s Arsacids and Sasanians (2011) are the modern treatments I find most useful for the political-ideological dimension; C. S. Lightfoot’s “Trajan’s Parthian War and the Fourth-Century Perspective,” in Journal of Roman Studies 80 (1990), is the article I most often return to on the Roman side.

53 BCE: Carrhae

The first great Parthian–Roman war is the one that ended at Carrhae. The destruction of Crassus’ army and the capture of the eagle standards are treated in detail in the Battle of Carrhae article. The immediate consequence was a long period of tension. Mark Antony, the most powerful Roman in the East after the death of Caesar, planned a major invasion of Parthia in 36 BCE. The campaign was a disaster. Antony’s forces were harassed by Parthian horse archers across the Armenian highlands, and the invasion ended in a negotiated withdrawal. The first Roman emperor, Augustus, signed a formal peace with the Parthian king Phraates IV in 20 BCE, in which the Romans recovered the captured eagle standards and the Parthians accepted the Roman annexation of Armenia. The treaty established a working relationship that lasted, with intervals, for nearly a century.

The Armenian settlement of 20 BCE was, in substance, a partition: the Roman candidate was confirmed on the throne, but a member of the Arsacid house was given a role in the kingdom, and the Roman candidate was required to be crowned at Rome before taking up his office. The arrangement, known in the sources as the adiectio, was a Roman innovation; it was meant to satisfy both the Roman idea of client kingship and the Parthian idea of dynastic legitimacy, and it satisfied neither. It was the source of almost every Armenian succession crisis of the next century.

Trajan (114–117 CE)

The next major war was launched by the Roman emperor Trajan in 114 CE. Trajan had already conquered Dacia in 106 CE and was at the height of his power. The pretext was the deposition by the Parthian king Osroes I of the Roman client king of Armenia and the installation of his own candidate. Trajan invaded Armenia, deposed the Parthian client, and annexed the country as a Roman province. He then turned south into Mesopotamia. Seleucia-on-the-Tigris fell in late 115, and Ctesiphon — the Parthian capital — was taken early in 116. Trajan pushed further south, reaching the Persian Gulf and reportedly wishing that he were younger so he could march on India the way Alexander had. Cassius Dio, in his Roman History 68.28, preserves the speech in which Trajan is said to have expressed the wish.

The conquests did not hold. The Parthian population revolted; the Roman garrisons were overstretched; and a serious outbreak of plague among the Roman troops complicated the situation further. Trajan was forced to withdraw from most of the conquests, and by the time of his death in August 117 the Roman position in the East was already collapsing. His successor Hadrian, who came to the throne within a month, made peace with the Parthians and withdrew to the traditional Euphrates frontier. The Roman province of Mesopotamia, briefly established, was abandoned. Lightfoot (1990) reads the Trajanic war as a strategic over-extension that the Roman state could not, in the long run, support; the consensus is that he is right.

Lucius Verus and the Antonine Plague (161–166 CE)

The next Parthian–Roman war was launched in 161 CE by the co-emperor Lucius Verus, in the name of his adoptive brother Marcus Aurelius. The pretext, as before, was the Parthian king Vologases IV’s intervention in Armenia. The Roman response was swift. The general Statius Priscus invaded Armenia and restored the Roman client king; Avidius Cassius then invaded Mesopotamia, took Seleucia, and in 165 or 166 took Ctesiphon. The Roman army sacked the royal palace and returned to Syria with an enormous booty.

The war’s most enduring consequence was not military. The Roman army brought back a plague — the so-called Antonine Plague, almost certainly smallpox, identified by Harper (2017) and earlier by Littman and Littman as a particularly severe strain of variola major — that swept through the Roman Empire and killed an estimated five to ten million people over the next fifteen years. The demographic shock is visible in the Roman epigraphic and papyrological record. The plague weakened the Roman state for decades and prevented the exploitation of the Ctesiphon victory. The Parthians, for their part, did not mount a serious counter-offensive; the empire under Vologases IV and his successors turned inward, preoccupied with internal succession struggles and the rise of Vologases’ own clients in Armenia and the Caucasus.

A small clarification, because popular literature confuses the two. The famous sack of Antioch in 256 CE — the most dramatic Iranian strike into Roman territory of the third century — was not a Parthian operation. The Arsacid dynasty had been replaced by the Sasanians in 224 CE, and the sack of Antioch in 256 CE was the work of Shapur I. The events belong to different empires, and the conflation should be avoided.

The Rhandeia settlement and its consequences (63 CE)

The single most important diplomatic event of the first century CE was the Treaty of Rhandeia in 63 CE, which was the first formal settlement of the Armenian question and the model for the next two hundred years. The Roman general Corbulo, in negotiation with the Parthian king Vologases I, agreed that Tiridates — an Arsacid prince, the brother of Vologases I — would travel to Rome to receive the Armenian crown from the Roman emperor Nero, and would rule Armenia as a Roman client. The coronation of Tiridates by Nero, in 66 CE, was a Roman ceremonial event of the first order, recorded in detail by Cassius Dio (61.3–7) and depicted on a relief in the Forum of Rome. The arrangement, in its essentials, held for the next century. It is worth noting that the Rhandeia settlement was, in substance, a return to the Augustan adiectio of 20 BCE; the Roman and Parthian empires had been arguing about how to combine Roman client-kingship with Parthian dynastic legitimacy for sixty years, and the Rhandeia formula was the most stable answer they could find.

Septimius Severus and the end (194–202 CE)

The last great Parthian–Roman war was launched by the Roman emperor Septimius Severus in 194 CE. Severus had come to the throne after the civil war of 193, and he was a determined, professional soldier. The pretext was the Parthian support for his rival Pescennius Niger, but the underlying motive was the same as Trajan’s: a Mesopotamian campaign was a way to consolidate Severan legitimacy and to gain glory. Herodian’s History of the Roman Empire since Marcus Aurelius (3.9) gives the official Roman account; Cassius Dio’s epitome (75.1–3) is more useful for the strategic context.

The campaign was a Roman victory. Severus invaded Mesopotamia in 194, took Nisibis, and in 195 occupied Ctesiphon. The Parthian king Vologases V fled. A second campaign in 202, against the Parthian king Pacorus, captured Seleucia and Babylon and led to the formal annexation of the Roman province of Mesopotamia. The new province was defended by a line of fortifications along the Khabur and the upper Tigris. The Roman presence in northern Mesopotamia was now intended to be permanent.

The Parthian state did not long survive the loss. The Sasanian revolution of 224 CE, led by Ardashir I from Persis, overthrew the last Arsacid king of kings, Artabanus IV, at the Battle of Hormizdagan, and replaced the Arsacid dynasty with the more centralised and militarily aggressive Sasanian house. The Roman wars with the Sasanians, which would last in various forms until the Arab conquests of the seventh century, were the continuation of the long Roman engagement with Iran.

Patterns and the question of decision

A pattern emerges. The Parthian–Roman wars were largely inconclusive at the strategic level: the Parthians could not project decisive force into Roman territory, and the Romans could not hold Mesopotamia against a determined Parthian counter-attack. The result was a long pattern of wars, treaties, and the gradual exhaustion of both sides. The most successful Roman campaigns — those of Trajan and Septimius Severus — produced spectacular short-term conquests that did not hold. The most successful Parthian operation — the defence against Crassus — was followed by decades of cautious diplomacy and the gradual recovery of the upper hand. The contest was, in a way that neither side quite intended, a balance-of-power equilibrium maintained by mutual exhaustion.

The economic dimension is part of the picture. The Parthian silver drachm, struck in declining fineness from Mithridates I onward, was the principal currency of the Near East, and Roman senators and emperors were major importers of Parthian silk, Parthian textiles, and the products of the Parthian caravan cities. The Parthian state was, in turn, a major exporter of luxury goods and a major consumer of Roman glass, wine, and coin. The political stability of the Parthian empire was, for that reason, an economic interest of Rome. The Sasanians, who inherited this position, would prove considerably more aggressive in their management of it. The Roman–Parthian wars were fought on the same ground as the Roman–Parthian trade, and the two could not be fully separated.

The military legacy

The Parthian wars forced the Roman army to change. The legion, the basic Roman heavy-infantry unit, remained the centre of Roman military power, but the Roman army of the second and third centuries increasingly incorporated the lessons of the eastern wars: more cavalry, more mounted archers, more flexibility in tactical deployment. The Roman emperor Hadrian, a serious student of the Parthian military system, restructured the auxilia along lines that owed something to the Arsacid model. The cataphract, the four-horned saddle, and the composite bow would become standard equipment of the late Roman and Byzantine army, and the Sasanian field engineering of late antiquity — mining, siege towers, the organised field camp — would be a major influence on Byzantine practice.

What is less often noted is the Iranian side of the exchange. The Parthian kings of the second century CE adopted Roman titles, Roman architectural forms, and Roman artistic motifs; the Parthian coinage, in particular, was deeply influenced by Roman models. The cultural exchange was substantial, and the Sasanian revolution of 224 CE can be read, in part, as a national reaction against the Romanised Arsacid house.

The verdict

The Parthian–Roman wars were the central political fact of the ancient Near East for nearly three centuries. They did not decide the contest. The Sasanians would inherit the rivalry, and the contest with the Sasanians, with greater intensity, would last for another four centuries. The final word on the Roman–Iranian contest belongs, in a way, to Heraclius, who in 628 CE recovered the lost Sasanian conquests and then watched the Arab armies walk into the vacuum.