In the spring of A.D. 195, Severus, at the head of his troops, crossed the Euphrates in person, and taking up his own quarters at Nisibis, which the Mesopotamians had been unable to capture, proceeded to employ his generals in the reduction of the rebels and the castigation of their aiders and abettors. Though his men suffered considerably from the scarcity and badness of the water, yet he seems to have found no great difficulty in reducing Mesopotamia once more into subjection. Having brought it completely under, and formally made Nisibis the capital, at the same time raising it to the dignified position of a Roman colony, he caused his troops to cross the Tigris into Adiabene, and, though the inhabitants offered a stout resistance, succeeded in making himself master of the country. The Parthian monarch seems to have made no effort to prevent the occupation of this province. He stood probably on the defensive, expecting to be attacked, in or near his capital. But Severus could not afford to remain in these remote regions. He had still a rival in the West in the person of Clodius Albinus, who might be expected to descend upon Italy, if it were left exposed to his attacks much longer. He therefore quitted the East early in A.D. 196, and returned to Rome with all speed, leaving Parthia very insufficiently chastised, and his new conquests very incompletely settled.
Scarcely was he gone when the war broke out with greater violence than ever. Volagases took the offensive, recovered Adiabene, and crossing the Tigris into Mesopotamia, swept the Romans from the open country. Nisibis alone, which two years before had defied all the efforts of the Mesopotamians, held out against him, and even this stronghold was within a little of being taken. According to one writer, the triumphant Parthians even crossed the Euphrates, and once more spread themselves over the fertile plains of Syria. Severus was forced in A.D. 197 to make a second Eastern expedition to recover his lost glory and justify the titles which he had taken. On his first arrival in Syria, he contented himself with expelling the Parthians from the province, nor was it till late in the year, that, having first made ample preparation, he crossed the Euphrates into Mesopotamia.
The success of any expedition against Parthia depended greatly on the dispositions of the semi-dependent princes, who possessed territories bordering upon those of the two great empires. Among these the most important were at this time the kings of Armenia and Osrhoene. Armenia had at the period of Niger’s attempt been solicited by his emissaries; but its monarch had then refused to take any part in the civil conflict. Subsequently, however, he in some way offended Severus who, when he reached the East, regarded Armenia as a hostile State requiring instant subjugation. It seems to have been in the summer of A.D. 197, soon after his first arrival in Syria, that Severus despatched a force against the Armenian prince, who was named (like the Parthian monarch of the time) Volagases. That prince mustered his troops and met the invaders at the frontier of his kingdom. A battle seemed imminent; but ere the fortune of war was tried the Armenian made an application for a truce, which was granted by the Roman leaders. A breathing-space being thus gained, Volagases sent ambassadors with presents and hostages to the Roman emperor in Syria, professed to be animated by friendly feelings towards Rome, and entreated Severus to allow him terms of peace. Severus permitted himself to be persuaded; a formal treaty was made, and the Armenian prince even received an enlargement of his previous territory at the hands of his mollified suzerain.
The Osrhoenian monarch, who bore the usual name of Abgarus, made a more complete and absolute submission. He came in person into the emperor’s camp, accompanied by a numerous body of archers, and bringing with him his sons as hostages. Severus must have hailed with especial satisfaction the adhesion of this chieftain, which secured him the undisturbed possession of Western Mesopotamia as far as the junction of the Khabour with the Euphrates. It was his design to proceed himself by the Euphrates route, while he sent detachments under other leaders to ravage Eastern Mesopotamia and Adiabene, which had evidently been re-occupied by the Parthians. To secure his army from want, he determined, like Trajan, to build a fleet of ships in Upper Mesopotamia, where suitable timber abounded, and to march his army down the left bank of the Euphrates into Babylonia, while his transports, laden with stores, descended the course of the river. In this way he reached the neighborhood of Ctesiphon without suffering any loss, and easily captured the two great cities of Babylon and Seleucia, which on his approach were evacuated by their garrisons. He then proceeded to the attack of Ctesiphon itself, passing his ships probably through one of the canals which united the Tigris with the Euphrates, or else (like Trajan) conveying them on rollers across the neck of land which separates the two rivers.
Volagases had taken up his own position at Ctesiphon, bent on defending his capital. It is possible that the approach of Severus by the line of march which he pursued was unexpected, and that the sudden presence of the Romans before the walls of Ctesiphon came upon the Parthian monarch as a surprise. He seems, at any rate, to have made but a poor resistance. It may be gathered, indeed, from one author that he met the invaders in the open field, and fought a battle in defence of Ctesiphon before allowing himself to be shut up within its walls. But after the city was once invested it appears to have been quickly taken. We hear of no such resistance as that which was soon afterwards offered by Hatra. The soldiers of Severus succeeded in storming Ctesiphon on the first assault; the Parthian monarch betook himself to flight, accompanied by a few horsemen; and the seat of empire thus fell easily—a second time within the space of eighty-two years—into the hands of a foreign invader. The treatment of the city was such as we might expect from the ordinary character of Roman warfare. A general massacre of the male population was made. The soldiers wore allowed to plunder both the public and the private buildings at their pleasure. The precious metals accumulated in the royal treasury were seized, and the chief ornaments of the palace were taken and carried off. Nor did blood and plunder content the victors. After slaughtering the adult males they made prize of the women and children, who were torn from their homes without compunction and led into captivity, to the number of a hundred thousand.
Notwithstanding the precautions which he had taken, Severus appears to have become straitened for supplies about the time that he captured Ctesiphon. His soldiers were compelled for some days to exist on roots, which produced a dangerous dysentery. He found himself unable to pursue Volagases, and recognized the necessity of retreating before disaster overtook him. He could not, however, return by the route of the Euphrates, since his army had upon its advance completely exhausted the resources of the Euphrates region. The line of the Tigris was therefore preferred for the retreat; and while the ships with difficulty made their way up the course of the stream, the army pursued its march upon the banks, without, so far as appears, any molestation. It happened, however, that the route selected led Severus near to the small state of Hatra, which had given him special offence by supporting the cause of his rival, Niger; and it seemed to him of importance that the inhabitants should receive condign punishment for this act of audacity. He may also have hoped to eclipse the fame of Trajan by the capture of a town which had successfully resisted that hero. He therefore stopped his march in order to lay siege to the place, which he attacked with military engines, and with all the other offensive means known at the time to the Romans. His first attempt was, however, easily repulsed. The walls of the town were strong, its defenders brave and full of enterprise. They burnt the siege-machines brought against them, and committed great havoc among the soldiers. Under these circumstances disorders broke out among the besiegers; mutinous words were heard; and the emperor thought himself compelled to have recourse to severe measures of repression. Having put to death two of his chief officers, and then found it necessary to deny that he had given orders for the execution of one of them, he broke up from before the place and removed his camp to a distance.
He had not, however, as yet relinquished the hope of bringing his enterprise to a successful issue. In the security of his distant camp he constructed fresh engines in increased numbers, collected an abundant supply of provisions, and made every preparation for renewing the siege with effect at no remote period. The treasures stored up in the city were reported to be great, especially those which the piety of successive generations had accumulated in the Temple of the Sun. This rich booty appealed forcibly to the cupidity of the emperor, while his honor seemed to require that he should not suffer a comparatively petty town to defy his arms with impunity. He, therefore, after a short absence retraced his steps, and appeared a second time before Hatrawith a stronger siege-train and a better appointed army than before. But the Hatreni met his attack with a resolution equal to his own. They were excellent archers; they possessed a powerful force of cavalry; they knew their walls to be strong; and they were masters of a peculiar kind of fire, which was calculated to terrify and alarm, if not greatly to injure, an enemy unacquainted with its qualities. Severus once more lost almost all his machines; the Hatrene cavalry severely handled his foragers; his men for a long time made but little impression upon the walls, while they suffered grievously from the enemy’s slingers and archers, from his warlike engines, and especially, we are told, from the fiery darts which were rained upon them incessantly. However, after enduring these various calamities for a length of time, the perseverance of the Romans was rewarded by the formation of a practicable breach in the outer wall; and the soldiers demanded to be led to the assault, confident in their power to force an entrance and carry the place. But the emperor resisted their inclination. He did not wish that the city should be stormed, since in that case it must have been given up to indiscriminate pillage, and the treasures which he coveted would have become the prey of the soldiery. The Hatreni, he thought, would make their submission, if he only gave them a little time, now that they must see further resistance to be hopeless. He waited therefore a day, expecting an offer of surrender. But the Hatreni made no sign, and in the night restored their wall where it had been broken down.
Severus then made up his mind to sacrifice the treasures on which his heart had been set, and, albeit with reluctance, gave the word for the assault. But now the legionaries refused. They had been forbidden to attack when success was certain and the danger trivial—they were now required to imperil their lives while the result could not but be doubtful. Perhaps they divined the emperor’s motive in withholding them from the assault, and resented it; at any rate they openly declined to execute his orders. After a vain attempt to force an entrance by means of his Asiatic allies, Severus desisted from his undertaking. The summer was far advanced the heat was great; disease had broken out among his troops; above all, they had become demoralized, and their obedience could no longer be depended on. Severus broke up from before Hatra a second time, after having besieged it for twenty days, and returned—by what route we are not told—into Syria.
Nothing is more surprising in the history of this campaign than the inaction and apparent apathy of the Parthians. Volagases, after quitting his capital, seems to have made no effort at all to hamper or harass his adversary. The prolonged resistance of Hatra, the sufferings of the Romans, their increasing difficulties with respect to provisions, the injurious effect of the summer heats upon their unacclimatized constitutions, would have been irresistible temptations to a prince of any spirit or energy, inducing him to advance as the Romans retired, to hang upon their rear, to cut off their supplies, and to render their retreat difficult, if not disastrous. Volagases appears to have remained wholly inert and passive. His conduct is only explicable by the consideration of the rapid decline which Parthia was now undergoing, of the general decay of patriotic spirit, and the sea of difficulties into which a monarch was plunged who had to retreat before an invader.
The expedition of Severus was on the whole glorious for Rome, and disastrous for Parthia, though the glory of the victor was tarnished at the close by his failure before Hatra. It cost Parthia a second province. The Roman emperor not only recovered his previous position in Mesopotamia, but overstepping the Tigris, established the Roman dominion firmly in the fertile tract between that stream and the Zagros mountain-range. The title of “Adiabenicus” became no empty boast. Adiabene, or the tract between the Zab rivers—probably including at this time the entire low region at the foot of Zagros from the eastern Khabour on the north to the Adhem towards the south—passed under the dominion of Rome, the monarch of the country, hitherto a Parthian vassal, becoming her tributary. Thus the imperial standards were planted permanently at a distance less than a degree from the Parthian capital, which, with the great cities of Seleucia and Babylon in its neighborhood, was exposed to be captured almost at any moment by a sudden and rapid inroad.