On discovering this obstacle, Antoninus suggested that, by a march to the north-east through a fertile country, the "Upper Euphrates" might be reached, and easily crossed, before its waters had attained any considerable volume.

Sapor agreed to adopt this suggestion.
He marched from Zeugma across the Mons Masius towards the Upper Euphrates, defeated the Romans in an important battle near Arnida, took, by a sudden assault, two castles which defended the town, and then somewhat hastily resolved that he would attack the place, which he did not imagine capable of making much resistance.
Amida, now Diarbekr, was situated on the right bank of the Upper Tigris, in a fertile plain, and was washed along the whole of its western side by a semi-circular bend of the river.

It had been a place of considerable importance from a very ancient date, and had recently been much strengthened by Constantius, who had made it an arsenal for military engines, and had repaired its towers and walls.

The town contained within it a copious fountain of water, which was liable, however, to acquire a disagreeable odor in the summer time.

Seven legions, of the moderate strength to which legions had been reduced by Constantine, defended it; and the garrison included also a body of horse-archers, composed chiefly or entirely of noble foreigners.