The Parthian campaigns of Septimius Severus (195-198) involved the Roman armies' success over the Parthians for supremacy over the nearby Kingdom of Armenia. After this defeat the Parthians were first defeated by the Roman armies of Severus's son, Caracalla (215–217), and then replaced in 224 by the Sassanid dynasty.
Historial context.
Prelude.
The Severan dynasty, which reigned over the Roman Empire between the end of the 2nd and the first decades of the 3rd century, from 193 to 235, with a brief interruption during the reign of Macrinus between 217 and 218, had its founder in Septimius Severus and its last descendant in Alexander Severus. The new dynasty was born from the ashes of a long period of civil wars (Year of the Five Emperors), where three other contenders faced each other in addition to Septimius Severus (Didius Julianus, Pescennius Niger and Clodius Albinus). Furthermore, in the nomination of the emperors there was a clear reference to the Antonine dynasty. The reason was to create a form of ideal continuity with the previous dynasty, almost as if there had been no interruption, not even with his predecessor Pertinax.
"Casus belli".
Septimius Severus decided to invade Osroene in 195, since the Parthians had helped during 194 his direct rival to the imperial throne, Pescennius Niger, who had been defeated in three battles (at Cyzicus, Nicaea and Issus), and in an attempt to take refuge among the Parthians and fleeing from Antioch, was reached and killed. His head eventually found its way to Rome where it was displayed.
Forces in the field.
Roman forces.
The operations of these years of war beyond that allowed the emperor himself to constitute three new legions:
directly involved others such as:
in addition to some "vexillationes" coming from other fronts such as:
The total forces deployed by the Roman Empire may have exceeded 150,000 men involved; of these, half were legionaries (from as many as 24-25 legions), the remainder were "auxiliaries".
Parthian forces.
The Parthian units that took part in Septimius Severus's campaigns between 194 (or 195) and 198 AD are unknown.
Course of the campaign.
First campaign (194–195).
Adiabene and Osroene had rebelled against Rome, laying siege to the city of Nisibis. However, when they learned that Severus had defeated and killed Pescennius Niger, they decided to ask for his forgiveness, although they were not willing to free the Roman garrisons taken from Niger. Indeed, they demanded that the Romans leave the rest of their country free. For this reason Severus did not hesitate to wage war against them. In fact, he left Antioch for the Euphrates, crossing it at Zeugma during that particularly hot summer, so much so that the Roman army risked losing numerous soldiers due to dehydration.
After an initial clash, he managed to liberate the city of Nisibis, which had evidently been Roman since the time of the campaigns of Lucius Verus. He then decided to divide the army into three more sections, sending his subordinates, Lateranus, Julius Laetus and Candidus in different directions to subdue all the cities that had previously rebelled. Once they returned after having achieved their objective, Severus divided the army again between Laetus, , and Probus and sent them against a certain "Arche", evidently a king of the area, perhaps belonging to the population of the Arabs of the fortified city of Hatra, besieged at least twice by Severus (also during the campaign of 197–198). 
Some of these scenes are represented in the first south-east panel of the triumphal arch located near the Curia Julia in the Roman Forum. At the end of the war operations he reunited the province of Mesopotamia (which included only Osroene and Adiabene) by placing two of the three new legions just created (the Legio I and the III "Parthica") as garrison there, under the leadership of a prefect of equestrian rank. For these successes he assumed the titles of "Adiabenicus" and "Arabicus" in 195 AD.
Second campaign (197–198).
In early 197 Severus left Rome and sailed to the east. He embarked at Brundisium and probably landed at the port of Aegeae in Cilicia, travelling on to Syria by land. He immediately gathered his army and crossed the Euphrates. Abgar IX, titular King of Osroene but essentially only the ruler of Edessa since the annexation of his kingdom as a Roman province, handed over his children as hostages and assisted Severus' expedition by providing archers.
The second campaign was conducted from the summer of 197 to the spring of 198.
The king of the Parthians, Vologases V, having learned that Severus was approaching Nisibis, decided to leave. In the meantime, the Roman emperor, having reached the city, now free from the siege, had an unexpected encounter. Cassius Dio recounts, in fact, that here he found an enormous wild boar, which had killed a Roman knight, who had tried in vain to kill it. It took the intervention of about thirty soldiers to capture it and bring it to Severus.
Cassius Dio's records also tell us how he defined the Parthians "lazy" and "weak" for always retreating when Severus "showed up".
Severus, having built a fleet, crossed the Euphrates with extremely fast ships, where he first reached Dura Europos, continued to Seleucia which he occupied, after having put to flight the cataphract cavalry of the Parthians. The advance continued with the capture of Babylon which shortly before had been abandoned by the enemy forces and, towards the end of the year, even the capital of the Parthians, Ctesiphon, was placed under siege. The city, now surrounded, tried in vain to resist the impressive military machine that the Roman emperor had managed to put together (about 150,000 armed men). When it was now close to capitulation, King Vologases V abandoned his men and fled towards the interior of his territories. The city was sacked and many of its inhabitants were killed by Roman soldiers or sold into slavery, as had happened in the past at the time of Trajan (in 116) and Lucius Verus's (in 165).
Cassius Dio reports, finally, that Severus decided shortly after, to place the city of Hatra under siege once again, taking with him large quantities of food and siege engines, but on this occasion it is said that, not only did he lose a large amount of money for the preparation of the expedition, but also numerous war machines (apart from those of a certain Priscus), and furthermore, the emperor himself, during an attack on the enemy walls, almost risked his life, finally deciding to withdraw definitively and go to Egypt. Following the general success of the campaign, however, he earned the title of "Parthicus maximus".
Result of the campaign.
Analysis.
Especially after 197, Severus marched through northern Mesopotamia, re-annexing it to the empire and placing at its head a prefect of equestrian rank. Shortly afterwards he led the army towards Ctesiphon, sacking it and reducing most of its inhabitants to slavery or also deporting them. During his time in the east, though, Severus also expanded the "Limes Arabicus", building new fortifications in the Arabian Desert from Basie to Dumatha.
References.
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