Severus' Parthian War
From Trajan to the death of Commodus
Following the end of Trajan's war Roman policy toward Parthia changed, keeping the peace on the border between the two superpowers. That peace failed in 162 with the outbreak of a new Parthian war. This war, in which ultimate command was invested in Lucius Verus (younger co-emperor of Marcus Aurelius), lasted till 166. Parthian eagerness to avenge their losses to Verus, combined with the ambitions of Rome's new emperor, contributed to the outbreak of the latest Roman-Parthian war.
Unlike Trajan's short-lived gains, Verus won actual lasting gains from Parthia. After 166 Rome took control over western Mesopotamia between the Euphrates and Khabur rivers. In Syria the frontier moved all the way to Dura Europos and Rome gained control over one of the caravan routes linking Palmyra and Mesopotamia. In 191 a new great king ascended the throne, Valaksh (Vologases) V, who desired to win back the lost territory. However, Parthian power was in decline and a war with Rome considered too dangerous. The assassination of Roman Emperor Commodus in 193 and the civil war that followed changed those calculations.
Niger and the Revolt of the Mesopotamian Kingdoms
Roman Mesopotamia threw its support behind a contender named Pescennius Niger. The Roman client states in the east followed suit. Parthia's western clients also backed Niger, and even Valaksh himself offered support. Niger turned them down, but then changed his mind when Septimius Severus emerged as the strongest contender and was acknowledged as emperor in Rome. Pleas for aid were made to the kingdoms east of the Euphrates. Osrhoene, Adiabene, Hatra, and the Scenite Arabs all decided to send troops. The other kingdoms opted to remain neutral. Valaksh made promises of support, but was careful to keep his own troops out of the fighting. Meanwhile the Parthians convinced Adiabene and Osrhoene to rebel. Severus defeated Niger in 194, who was killed trying to escape to Parthia.
Severus, clearly, was not well disposed toward Parthia from the beginning. A siege of Nisibis, viewed by the Romans as their dependent, by an army of Niger's former Mesopotamian allies did not help matters. An embassy was dispatched to Sever claiming they were acting in his best interest. They said Nisibis was harboring some remnants of Niger's army and promised to hand over whatever spoils remained from the city and any prisoners they found. In return they demanded independence, also refusing to surrender the territory they had conquered from Niger. Severus rejected this justification and began preparing for war.
Much like Trajan, Severus’ decision to go to war was multifaceted. It might have been simply wishing to gain greater renown. More strategically, Niger's headquarters at Byzantium remained in enemy hands. In that case, fighting a foreign enemy after a long civil war would have seemed wise. Also, fighting a common foe would have helped integrate Niger's legions with Severus' existing forces. Last, Osrhoene and Roman Mesopotamia were viewed as strategically vital to the defense of Syria and the east. Mesopotamian intransience about handing over captured territory threatened that defense. Severus would have wanted to remedy that situation and bring the border in line with his own ideas.
The Campaign of 195 and Valaksh's Counterattack
Anyway the Roman army crossed the border at Zeugma in late spring, 195. They soon went over the Euphrates into Osrhoene, annexing it. A procurator governor was appointed. The city of Edessa, however, was allowed to remain independent with a small supporting territory. King Abgar IX was left in power over this rump kingdom and did homage to the emperor. From Edessa the army advanced on Nisibis, facing severe desert hardships along the way. The siege was broken, the city becoming campaign headquarters for that year. Meanwhile an embassy from a people only identified as “the Arabians” was received and rejected by the emperor because the ruler did come in person to pay homage. Next Severus began sending out divisions to pacify the region under Claudius Candidus, T. Sextius Lateranus, and Julius Laetus. Once they returned a second army of three divisions was sent out under Cornelius Anullinus, Julius Laetus, and a certain Probus. This time around they fought in an area of Mesopotamia called Arche and reduced it. Nisibis was made the capital of the region and the army crossed the Tigris into Adiabene. After taking the capital of Arbela they returned to Nisibis. In late 195, word reached Severus that Byzantium had fallen. At the same time news arrived that Clodius Albinus had risen in revolt in the western provinces. Albinus was Severus' former colleague and last imperial rival. Most of the army hurried west, with a holding force left behind. Command was entrusted to Valerius Valerianus, a talented cavalry officer. The Senate voted the emperor the titles of Parthicus Arabicus and Parthicus Adiabenicus before he left.
With the departure of the main Roman force, Valaksh saw his chance to strike back in 196. He mobilized his army and swept into Roman Mesopotamia, crossing the Euphrates into Syria at the height of his advance. Armenia was conquered and Adiabene returned to Parthian influence. In a show of force, Narses, the pro-Roman king of Adiabene, was killed by drowning in the Greater Zab River. Meanwhile Nisibis was besieged and nearly fell, but held out thanks to a skillful defense (and quick arrival from modern France) by Laetus. At the height of his success Valaksh was forced to divert his forces east. The vassal kings of Media and Persis rose in revolt, dissatisfied with Arsacid rule. The great king was at first forced to retreat but later reformed his ranks and defeated his pursuers. A second victory followed during the return march west, but when the Parthians returned to the west their chance had passed.
The Campaign of 197 and the Sack of Ctesiphon
In early 197 Severus could returned to the east on the defeat of Albinus. This time around he planned for a much larger campaign, this time aimed at the Parthian Empire itself rather then just its client kingdoms in Mesopotamia. He arrived in summer, expelling the Parthians from Syria personally while sending a second army in Armenia to secure his flank. The Armenian king, Vagharsh (Armenian form of Valaksh), quickly sued for peace and was rewarded with an expanded kingdom. The emperor then returned west to raise fresh legions for the next offensive, set for later that year.
In preparation for the attack, the emperor created three new legions (Legions I, II, III Parthica) to bolster his army. The I and III Parthica (the II being left behind in Italy), as well as part of Legion III Augusta went on ahead to Antioch, crossing the Euphrates on Severus' arrival. Abgar of Edessa and Vagharsh of Armenia quickly reaffirmed their submission to Rome, with Osrhoene even contributing troops to the Roman army. The Parthians, still besieging Nisibis, withdrew before the emperor's advance and he returned to Syria. Severus now meant to emulate Trajan and Verus and sack Ctesiphon. To this end he built a river fleet to sail down the Euphrates. The advance began in earnest that September, with the river navy sailing south in Babylonia accompanied by a division marching along the left bank. He also sent out two other divisions. One was charged with pacifying eastern Mesopotamia again. The other crossed the Tigris to attack Adiabene. Joining the Romans for this advance was an unidentified brother of Valaksh V. Babylon and Seleucia were found abandoned by the invaders, leaving the capital wide open. The arrival of the Romans caught Valaksh by surprise. He attempted to face Severus on the open field outside Ctesiphon, but was defeated. A short siege followed, and a Roman assault took the walls. The great king fled to the east and his capital was sacked and plundered. This was the third time Roman troops had taken the city in the last 80 years.
The First Siege of Hatra
Severus celebrated his accomplishment by taking the title of Parthicus Maximus on the centennial of Trajan's accession, January 28th, 198. Severus now found himself faced with several problems. Despite his best efforts food supplies were scarce and his military intelligence poor. The decision was made to retreat before the Parthians could take advantage of Roman weakness. The army left Ctesiphon by way of the Tigris, hoping to gather supplies along a route they had not taken before. The emperor had additional reasons for choosing this route. Marching north along the Tigris would take the Romans past Hatra, which had thus far remained untouched in spite their previous support for Niger. Hatra could also pose a threat to the retreating troops passing along the river if they chose to. Also important to Severus, the city had held out against Trajan, so taking it would allow him to surpass his glorious predecessor. From a wider perspective, Hatra needed to be taken to complete a chain of communication and defense between the Euphrates and Tigris connecting Dura Europos, Nisibis, and Singara.
The siege, lasting from February to early spring, was a dismal failure. Hatra was well defended and surrounded by massive double walls stretching over four miles. Every attempt to breach the walls was met by Hatrene arrows. The dead and wounded mounted and Severus executed a Praetorian tribune for complaining of the pointlessness of the siege. Julius Laetus was also executed despite of his military skill. The emperor feared his general had become too popular with the troops in the face of his own growing unpopularity. For that reason he even tried to blame the army itself for Laetus' death. The siege camp was moved further away from the walls and new supplies gathered. Preparations were made for a renewed attack on Hatra, but finally Severus quit the siege and completed the retreat to Nisibis. He had nothing to show for it save a bruised ego.
After this failure the emperor turned his attention toward consolidation. The province of Osrhoene was refined and Carrhae was made the capital. A new, formal, province of Mesopotamia was created from the territory east of the rump kingdom of Edessa with its capital at Nisibis. Severus intended for Mesopotamia to be the lynchpin of his eastern defense strategy, and placed it under an equestrian prefect like Egypt had.
The Second Siege of Hatra and the end of Severus' campaigns
But Hatra still remained as an embarrassment to Severus. The new borders now put the small kingdom just over sixty miles from Roman territory. By the end of 198 the Roman army in the east consisted of a powerful Danubian and Praetorian core. In Osrhoene and Mesopotamia the garrisons were strengthened in case of invasion. A skilled siege engineer from Bithynia, Priscus, now served the emperor after having frustrated his armies in the service of Niger during the long siege of Byzantium. Hatra was placed under siege for the second time in the early months of 199.
Remembering the harsh lessons of before, the Romans took greater precautions for the second siege. A great store of supplies was prepared for a long siege and better quality siege engines were built in advance, as well as some special machines just for this occasion built by Priscus. But the Hatrene king, Abdsemya (Barsemius or Barsemias), had made his own preparations. Hatrene archery remained effective, now with the support of artillery (one salvo killed several of Severus' guards). Troops approaching the city were forced to deal with flaming asphalt (called “Hatrene fire”) and jars of stinging insects flung from the walls. The defenders also made excellent use of their Parthian style cavalry, destroying Roman forage parties and siege engines. However, the Romans eventually breached the outer walls, only to be ordered to retreat rather than press the assault. The emperor was making a gamble the Hatrenes would take advantage of his show of benevolence. A peaceful surrender would allow them to avoid enslavement and spare the great temple of Maren Shamash (a solar deity) for which Hatra was famous from sack. Abdsemya had no intention of surrendering and during the night of the one day grace period had his soldiers repair the breach. The following morning Severus, now frustrated, ordered a new assault on the walls. The European troops refused, furious because they had been forced to retreat the previous day. The Syrians replaced them but were slaughtered and forced to fall back. After 20 days of fighting the emperor quit the siege, retreating back to Syria again, ending his Parthian war.
Twice now Septimius Severus had failed to take Hatra, but unlike Trajan his campaigns did not end in total failure. Abdsemya submitted to Roman authority shortly after the second siege and a Roman garrison installed in Hatra, accomplishing his objective. Both campaigns had been largely defensive. The first was provoked by the refusal of the Mesopotamian client kings to return the territory they took from Niger. The second was a response to the Parthian sweep through Roman territory in 196. When the war ended Severus had his eastern bulwark, and was reorganizing the eastern frontiers more to his liking. Advancing beyond the eastern bank of the Tigris had never crossed his mind.