Things came to a conflict under Trajan. In the earlier years of his government he had made no essential change in eastern affairs, apart from the conversion of the two client-states hitherto subsisting on the border of the Syrian desert--the Nabataean of Petra and the Jewish of Caesarea Paneas--into administrative districts directly Roman (A.D. 106). The relations with the ruler of the Parthian kingdom at that time, king Pacorus, were not the most friendly,[46] but it was only under his brother and successor Chosroes that a rupture took place, and that again concerning Armenia. The Parthians were to blame for it. When Trajan bestowed the vacated throne of the Armenian king on Axidares the son of Pacorus, he kept within the limits of his right; but king Chosroes described this personage as incapable of governing, and arbitrarily installed in his stead another son of Pacorus, Parthomasiris, as king.[47] The answer to this was the Roman declaration of war. Trajan left the capital towards the end of the year 114,[48] to put himself at the head of the Roman troops of the East, which were certainly once more found in the deepest degeneracy, but were reorganised in all haste by the emperor, and reinforced besides by better legions brought up from Pannonia.[49]
Envoys of the Parthian king met him at Athens; but they had nothing to offer except the information that Parthomasiris was ready to accept Armenia as a Roman fief, and were dismissed. The war began. In the first conflicts on the Euphrates the Romans fared worst;[50] but when the old emperor, ready to fight and accustomed to victory, placed himself at the head of the troops in the spring of 115, the Orientals submitted to him almost without resistance. Moreover, among the Parthians civil war once more prevailed, and a pretender, Manisarus, had appeared against Chosroes. From Antioch the emperor marched to the Euphrates and farther northward as far as the most northerly legion-camp Satala in Lesser Armenia, whence he advanced into Armenia and took the direction of Artaxata. On the way Parthomasiris appeared in Elegeia and took the diadem from his head, in the hope of procuring investiture through this humiliation, as Tiridates had once done. But Trajan was resolved to make this vassal-state a province, and to shift the eastern frontier of the empire generally. This he declared to the Parthian prince before the assembled army, and directed him with his suite to quit at once the camp and the kingdom; thereupon a tumult took place, in which the pretender lost his life. Armenia yielded to its fate, and became a Roman governorship. The princes also of the Caucasian tribes, the Albani, the Iberi, farther on toward the Black Sea the Apsilae, the Colchi, the Heniochi, the Lazi, and various others, even those of the trans-Caucasian Sarmatae, were confirmed in the relation of vassalage, or now subjected to it. Trajan thereupon advanced into the territory of the Parthians and occupied Mesopotamia. Here, too, all submitted without a blow; Batnae, Nisibis, Singara came into the power of the Romans; in Edessa the emperor received not merely the subjection of Abgarus, the ruler of the land, but also that of the other dynasts, and, like Armenia, Mesopotamia became a Roman province. Trajan took up once more his winter quarters in Antioch, where a violent earthquake demanded more victims than the campaign of the summer. In the next spring (116) Trajan, the “victor of the Parthians,” as the senate now saluted him, advanced from Nisibis over the Tigris, and occupied, not without encountering resistance at the crossing and subsequently, the district of Adiabene; this became the third new Roman province, named Assyria. The march went onward down the Tigris to Babylonia; Seleucia and Ctesiphon fell into the hands of the Romans, and with them the golden throne of the king and his daughter; Trajan reached even the Persian satrapy of Mesene, and the great mercantile town at the mouth of the Tigris, Charax Spasinu. This region also seems to have been incorporated with the empire in such a way that the new province Mesopotamia embraced the whole region enclosed by the two rivers.