The bands of Sapor crossed the Roman frontier soon after, if not even before, the death of Constantine; and after an interval of forty years the two great powers of the world were once more engaged in a bloody conflict. Constantius, having paid the last honors to his father’s remains, hastened to the eastern frontier, where he found the Roman army weak in numbers, badly armed and badly provided, ill-disposed towards himself, and almost ready to mutiny. It was necessary, before anything could be done to resist the advance of Sapor, that the insubordination of the troops should be checked, their wants supplied, and their good-will conciliated. Constantius applied himself to effect these changes. Meanwhile Sapor set the Arabs and Armenians in motion, inducing the Pagan party among the latter to rise in insurrection, deliver their king, Tiranus, into his power, and make incursions into the Roman territory, while the latter infested with their armed bands the provinces of Mesopotamia and Syria. He himself was content, during the first year of the war, A.D. 337, with moderate successes, and appeared to the Romans to avoid rather than seek a pitched battle. Constantius was able, under these circumstances, not only to maintain his ground, but to gain certain advantages. He restored the direction of affairs in Armenia to the Roman party, detached some of the Mesopotamian Arabs from the side of his adversary, and attached them to his own, and even built forts in the Persian territory on the further side of the Tigris. But the gains made were slight; and in the ensuing year (A.D. 338) Sapor took the field in greater force than before, and addressed himself to an important enterprise. He aimed, it is evident, from the first, at the recovery of Mesopotamia, and at thrusting back the Romans from the Tigris to the Euphrates. He found it easy to overrun the open country, to ravage the crops, drive off the cattle, and burn the villages and homesteads. But the region could not be regarded as conquered, it could not be permanently held, unless the strongly fortified posts which commanded it, and which were in the hands of Rome, could be captured. Of all these the most important was Nisibis. This ancient town, known to the Assyrians as Nazibina, was, at any rate from the time of Lucullus, the most important city of Mesopotamia. It was situated at the distance of about sixty miles from the Tigris, at the edge of the Mons Masius, in a broad and fertile plain, watered by one of the affluents of the river Khabour, or Aborrhas. The Romans, after their occupation of Mesopotamia, had raised it to the rank of a colony; and its defences, which were of great strength, had always been maintained by the emperors in a state of efficiency. Sapor regarded it as the key of the Roman position in the tract between the rivers, and, as early as A.D. 338, sought to make himself master of it.