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.