The Roman–Persian wars, also called the Roman–Iranian wars, took place between the Roman states and the Iranian empires, beginning with the Roman Republic and the Parthian Empire in 54 BC and ending with the Byzantine and the Sasanian empires in 628 AD. While the conflict between the two civilizations did involve direct military engagements, a significant role was played by a plethora of vassal kingdoms and allied nomadic tribes, which served as buffer states or proxies for either side. Despite nearly seven centuries of hostility, the Roman–Persian wars had an entirely inconclusive outcome, as both the Byzantines and the Sasanians were attacked by the Rashidun Caliphate as part of the early Muslim conquests. The Arab-Muslim offensives resulted in the collapse of the Sasanian Empire and largely confined the Byzantine Empire to Anatolia and southeastern Europe for the ensuing Arab–Byzantine wars.
Aside from shifts in the north, the Roman–Persian border remained largely stable for the duration of the conflict, albeit subject to an effective tug of war: towns, fortifications, and provinces were continually sacked, captured, destroyed, and traded, but neither side had the logistical strength or manpower to maintain such lengthy campaigns far from their borders, and thus neither could advance too far without risking stretching their frontiers too thin. Both sides did make conquests beyond the border, but in time, the balance was almost always restored. Although initially different in military tactics, the Romans and the Persians gradually adopted from each other, and by the second half of the 6th century, they were similar and evenly matched.
Ultimately, the expense of resources during the Roman–Persian wars proved catastrophic for both sides, as the prolonged and escalating warfare of the 6th and 7th centuries left them militarily exhausted and vulnerable in the face of the sudden emergence and expansion of the Rashidun army. Benefiting from the two empires' weakened conditions, the Rashidun Caliphate swiftly conquered Persia on the Sasanian front; and the Levant, the Caucasus, and Egypt and the rest of North Africa on the Byzantine front.
Background
According to James Howard-Johnston, "from the third century BC to the early seventh century AD, the rival players [in the East] were grand polities with imperial pretensions, which had been able to establish and secure stable territories transcending regional divides". The Romans and Parthians came into contact through their respective conquests of parts of the Seleucid Empire. During the 3rd century BC, the Parthians migrated from the Central Asian steppe into northern Iran and established an independent state that steadily expanded at the expense of the Seleucids, and through the course of the 2nd and early 1st centuries BC, they had conquered Persia, Mesopotamia, and Armenia. Ruled by the Arsacid dynasty, the Parthians fended off several Seleucid attempts to regain their lost territories, and established several eponymous branches in the Caucasus, namely the Arsacid dynasty of Armenia, the Arsacid dynasty of Iberia, and the Arsacid dynasty of Caucasian Albania. Meanwhile, the Romans expelled the Seleucids from their territories in Anatolia in the early 2nd century BC, following the defeat of Antiochus III the Great at Thermopylae and Magnesia. Finally, in 64 BC, Pompey conquered the remaining Seleucid territories in Syria, extinguishing their state and advancing the Romans' eastern frontier to the Euphrates, where it met the territory of the Parthians.
Roman–Parthian wars
Roman Republic vs. Parthia
Parthian enterprise in the West began in the time of Mithridates I and was revived by Mithridates II, who negotiated unsuccessfully with Lucius Cornelius Sulla for a Roman–Parthian alliance (c. 105 BC). When Lucullus invaded Southern Armenia and led an attack against Tigranes in 69 BC, he corresponded with Phraates III to dissuade him from intervening. Although the Parthians remained neutral, Lucullus considered attacking them. In 66–65 BC, Pompey reached an agreement with Phraates, and Roman–Parthian troops invaded Armenia, but a dispute soon arose over the Euphrates boundary. Finally, Phraates asserted his control over Mesopotamia, except for the western district of Osroene, which became a Roman dependency.
The Roman general Marcus Licinius Crassus led an invasion of Mesopotamia in 53 BC with catastrophic results; he and his son Publius were killed at the Battle of Carrhae by the Parthians under General Surena; this was the worst Roman defeat since the battle of Arausio. The Parthians raided Syria the following year, and mounted a major invasion in 51 BC, but their army was caught in an ambush near Antigoneia by the Romans, and they were driven back.
The Parthians largely remained neutral during Caesar's civil war, fought between forces supporting Julius Caesar and forces supporting Pompey and the traditional faction of the Roman Senate. However, they maintained relations with Pompey, and after his defeat and death, a force under Pacorus I assisted the Pompeian general Q. Caecilius Bassus, who was besieged at Apamea Valley by Caesarian forces. With the civil war over, Julius Caesar prepared a campaign against Parthia, but his assassination averted the war. The Parthians supported Brutus and Cassius during the ensuing Liberators' civil war and sent a contingent to fight on their side at the Battle of Philippi in 42 BC. After the Liberators' defeat, the Parthians invaded Roman territory in 40 BC in conjunction with the Roman Quintus Labienus, a former supporter of Brutus and Cassius. They swiftly overran the Roman province of Syria and advanced into Judea, overthrowing the Roman client Hyrcanus II and installing his nephew Antigonus. For a moment, the whole of the Roman East seemed lost to the Parthians or about to fall into their hands. However, the conclusion of the second Roman civil war soon revived Roman strength in Asia. Mark Antony had sent Ventidius to oppose Labienus, who had invaded Anatolia. Soon Labienus was driven back to Syria by Roman forces, and, although reinforced by the Parthians, was defeated, taken prisoner, and killed. After suffering a further defeat near the Syrian Gates, the Parthians withdrew from Syria. They returned in 38 BC but were decisively defeated by Ventidius, and Pacorus was killed. In Judaea, Antigonus was ousted with Roman help by Herod in 37 BC. With Roman control of Syria and Judaea restored, Mark Antony led a huge army into Atropatene, but his siege train and its escort were isolated and wiped out, while his Armenian allies deserted. Failing to make progress against Parthian positions, the Romans withdrew with heavy casualties. Antony was again in Armenia in 33 BC to join with the Median king against Octavian and the Parthians. Other preoccupations obliged him to withdraw, and the whole region came under Parthian control.
Roman Empire vs. Parthia
With tensions between the two powers threatening renewed war, Octavian and Phraataces worked out a compromise in 1 AD. According to the agreement, Parthia undertook to withdraw its forces from Armenia and to recognize a de facto Roman protectorate there. Nonetheless, Roman–Persian rivalry over control and influence in Armenia continued unabated for the next several decades. War erupted in 58 AD, after the Parthian King Vologases I forcibly installed his brother Tiridates on the Armenian throne. Roman forces overthrew Tiridates and replaced him with a Cappadocian prince, triggering an inconclusive war. This came to an end in 63 AD after the Romans agreed to allow Tiridates and his descendants to rule Armenia on condition that they receive the kingship from the Roman emperor.
A fresh series of conflicts began in the 2nd century AD, during which the Romans consistently held the upper hand over Parthia. The Emperor Trajan invaded Armenia and Mesopotamia during 114 and 115 and annexed them as Roman provinces. He captured the Parthian capital, Ctesiphon, before sailing downriver to the Persian Gulf. However, uprisings erupted in 115 AD in the occupied Parthian territories, while a major Jewish revolt broke out in Roman territory, severely stretching Roman military resources. Parthian forces attacked key Roman positions, and the Roman garrisons at Seleucia, Nisibis and Edessa were expelled by the local inhabitants. Trajan subdued the rebels in Mesopotamia, but having installed the Parthian prince Parthamaspates on the throne as a client ruler, he withdrew his armies and returned to Syria. Trajan died in 117, before he was able to reorganize and consolidate Roman control over the Parthian provinces.
Trajan's Parthian War initiated a "shift of emphasis in the 'grand strategy of the Roman empire' ", but his successor, Hadrian, decided that it was in Rome's interest to re-establish the Euphrates as the limit of its direct control. Hadrian returned to the status quo ante, and surrendered the territories of Armenia, Mesopotamia, and Adiabene to their previous rulers and client-kings.
War over Armenia broke out again in 161, when Vologases IV defeated the Romans there, captured Edessa and ravaged Syria. In 163 a Roman counter-attack under Statius Priscus defeated the Parthians in Armenia and installed a favored candidate on the Armenian throne. The following year Avidius Cassius invaded Mesopotamia, winning battles at Dura-Europos and Seleucia and sacking Ctesiphon in 165. An epidemic which was sweeping Parthia at the time, possibly of smallpox, spread to the Roman army and forced its withdrawal; this was the origin of the Antonine Plague that raged for a generation throughout the Roman Empire. In 195–197, a Roman offensive under the Emperor Septimius Severus led to Rome's acquisition of northern Mesopotamia as far as the areas around Nisibis, Singara and the third sacking of Ctesiphon. A final war against the Parthians was launched by the Emperor Caracalla, who sacked Arbela in 216. After his assassination, his successor, Macrinus, was defeated by the Parthians near Nisibis. In exchange for peace, he was obliged to pay for the damage caused by Caracalla.
Roman–Sasanian wars
Early Roman–Sasanian conflicts
Conflict resumed shortly after the overthrow of Parthian rule and Ardashir I's foundation of the Sasanian Empire. Ardashir (r. 226–241) raided Mesopotamia and Syria in 230 and demanded the cession of all the former territories of the Achaemenid Empire. After fruitless negotiations, Alexander Severus set out against Ardashir in 232. One column of his army marched into Armenia, while two other columns operated in the south but failed and the campaign ended inconclusively. In 238–240, towards the end of his reign, Ardashir attacked again. More successful than during the earlier clash, the Persian forces took several cities in Syria and Mesopotamia, including Carrhae, Nisibis and Hatra.
The struggle resumed and intensified under Ardashir's successor Shapur I; he invaded Mesopotamia and captured Hatra, a buffer state which had recently shifted its loyalty, but his forces were defeated at a battle near Resaena in 243; Carrhae and Nisibis were retaken by the Romans. Encouraged by this success, the emperor Gordian III advanced down the Euphrates but was defeated near Ctesiphon in the Battle of Misiche in 244. Gordian either died in the battle or was murdered by his own men; Philip became emperor, and paid 500,000 denarii to the Persians in a hastily negotiated peace settlement.
With the Roman Empire weakened by Germanic invasions and a series of short-term emperors, Shapur I soon resumed his attacks. In the early 250s, Philip was involved in a struggle over the control of Armenia; Shapur conquered Armenia and killed its king, defeated the Romans at the Battle of Barbalissos in 252, then probably took and plundered Antioch. Between 258 and 260, Shapur captured Emperor Valerian after defeating his army at the Battle of Edessa. He advanced into Anatolia but was defeated by Roman forces there; attacks from Odaenathus of Palmyra forced the Persians to withdraw from Roman territory, surrendering Cappadocia and Antioch. Between 262 and 264 CE, Odaenathus launched systematic campaigns into Persian territory, recovering the strategic cities of Nisibis, Edessa and Carrhae and the entirety of Roman Mesopotamia, possibly even besieging, but failing to take, Ctesiphon. Odaenathus's final campaign, which took place between 266 and 267 AD, may have further encroached upon the territory of the Sassanid Empire, perhaps even occupying Armenia.
In 275 and 282 Aurelian and Probus respectively planned to invade Persia, but they were both murdered before they were able to fulfil their plans. In 283, the emperor Carus launched an invasion of Persia, sacking its capital, Ctesiphon; he would probably have extended his campaign if the Emperor had not died in December of the same year. His successor Numerian was forced to retreat.
After a brief period of peace during Diocletian's early reign, Narseh renewed hostilities with the Romans, invading Armenia and defeating Galerius not far from Carrhae in 296 or 297. However, in 298, Galerius defeated Narseh at the Battle of Satala, sacked the capital Ctesiphon, and captured the Persian treasury and royal harem. The resulting peace settlement gave the Romans control of the area between the Tigris and the Greater Zab. The Roman victory was the most decisive for many decades: all the territories that had been lost, all the debatable lands, and control of Armenia lay in Roman hands. Many cities east of the Tigris were given to the Romans, including Tigranokert, Saird, Martyropolis, Balalesa, Moxos, Daudia, and Arzen. Also, control of Armenia was given to the Romans.