The emperor Severus pursued the same course further and more decidedly. The year of the three emperors, 193, had led to the war between the legions of the West and those of the East, and with Pescennius Niger the latter had succumbed. The Roman vassal-princes of the East, and as well the ruler of the Parthians, Vologasus V., son of Sanatrucius, had, as was natural, recognised Niger, and even put their troops at his disposal; the latter had at first gratefully declined, and then, when his cause took a turn to the worse, invoked their aid. The other Roman vassals, above all the prince of Armenia, cautiously kept back; only Abgarus, the prince of Edessa, sent the desired contingent. The Parthians promised aid, and it came at least from the nearest districts, from the prince Barsemias of Hatra in the Mesopotamian desert, and from the satrap of the Adiabeni beyond the Tigris. Even after Niger’s death (194) these strangers not merely remained in the Roman Mesopotamia, but even demanded the withdrawal of the Roman garrisons stationed there and the giving back of this territory.[66] [Sidenote: Province of Mesopotamia.] Thereupon Severus advanced into Mesopotamia and took possession of the whole extensive and important region. From Nisibis an expedition was conducted against the Arab prince of Hatra, which, however, did not succeed in taking the fortified town; even beyond the Tigris against the satrap of Adiabene the generals of Severus accomplished nothing of importance.[67] But Mesopotamia, _i.e._ the whole region between the Euphrates and Tigris as far as the Chaboras, became a Roman province, and was occupied with two legions newly created on account of this extension of territory. The principality of Edessa continued to subsist as a Roman fief, but was now no longer border-territory but surrounded by land directly imperial. The considerable and strong city of Nisibis, thenceforth called after the name of the emperor and organised as a Roman colony, became the capital of the new province and seat of the governor. After an important portion of territory had thus been torn from the Parthian kingdom, and armed force had been used against two satraps dependent on it, the great-king made ready with his troops to oppose the Romans. Severus offered peace, and ceded for Mesopotamia a portion of Armenia. But the decision of arms was thereby only postponed. As soon as Severus had started for the West, whither the complication with his co-ruler in Gaul recalled him, the Parthians broke the peace[68] and advanced into Mesopotamia; the prince of Osrhoene was driven out, the land was occupied, and the governor, Laetus, one of the most excellent warriors of the time, was besieged in Nisibis. He was in great danger, when Severus once more arrived in the East in the year 198, after Albinus had succumbed. Thereupon the fortune of war turned. The Parthians retreated, and now Severus took the offensive. He advanced into Babylonia, and won Seleucia