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.