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 On the contrary, they required that all Roman soldiers still in their country should be withdrawn from it, and that their independence should henceforth be respected.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, the petty kings of Osrhoene, Adiabene, and Hatra; but behind them loomed the massive form of the Parthian State, which was attacked through them, and could not be indifferent to their fortunes.