It was still his great object to drive the Romans from Mesopotamia; and with that object in view it continued to be his first wish to obtain possession of Nisibis.

Accordingly, having settled Armenian affairs to his liking, he made, in A.D.346, a second attack on the great city of Northern Mesopotamia, again investing it with a large body of troops, and this time pressing the siege during the space of nearly three months.

Again, however, the strength of the walls and the endurance of the garrison baffled him.

Sapor was once more obliged to withdraw from, before the place, having suffered greater loss than those whom he had assailed, and forfeited much of the prestige which he had acquired by his many victories.
It was, perhaps, on account of the repulse from Nisibis, and in the hope of recovering his lost laurels, that Sapor, in the next year but one, A.D.348, made an unusual effort.

Calling out the entire military force of the empire, and augmenting it by large bodies of allies and mercenaries, the Persian king, towards the middle of summer, crossed the Tigris by three bridges, and with a numerous and well-appointed army invaded Central Mesopotamia, probably from Adiabene, or the region near and a little south of Nineveh.