Parthia, and though, like so many other
Parthian dependencies, it possessed its native kings, cannot have been
in a position to engage in a great war without permission from the
Court of Ctesiphon. When, therefore, we find that Barsemius, the King
of Hatra, not only received the envoys of Niger favorably, but actually
sent to his aid a body of archers, we must understand that Volagases
sanctioned the measure. Probably he thought it prudent to secure the
friendship of the pretender whom he expected to be successful, but
sought to effect this in the way that would compromise him least if the
result of the struggle should be other than he looked for. The sending
of his own troops to the camp of Niger would have committed him
irretrievably; but the actions of a vassal monarch might with some
plausibility be disclaimed.

As the struggle between the two pretenders progressed in the early
months of A.D. 194, the nations beyond the Euphrates grew bolder,
and allowed themselves to indulge their natural feelings of hostility
towards the Romans. The newly subjected Mesopotamians flew to arms,
massacred most of the Roman detachments stationed about their country,
and laid siege to Nisibis, which since the cession Rome had made her
head-quarters. The natives of the region were assisted by their kindred
races across the Tigris, particularly by the people of Adiabene, who,
like the Arabs of Hatra, were Parthian vassals. Severus had no sooner
overcome his rival and slain him, than he hastened eastward with the
object of relieving the troops shut up in Nisibis, and of chastising the
rebels and their abettors. It was in vain that the Mesopotamians sought
to disarm his resentment by declaring that they had taken up arms in his
cause, and had been only anxious to distress and injure the partisans of
his antagonist. Though they sent ambassadors to him with presents, and
offered to make restitution of the Roman spoil still in their hands,
and of the Roman prisoners, it was observed that they said nothing about
restoring the strongholds which they had taken, or resuming the position
of Roman tributaries. On the contrary, they required that all Roman
soldiers still in their country should be withdrawn from it, and that
their independence should henceforth be respected. As Severus was not
inclined to surrender Roman territory without a contest, war was at once
declared. His immediate adversaries were of no great account, being, as
they were, th