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.
The first siege of Nisibis by Sapor lasted, we are told, sixty-three days.

Few particulars of it have come down to us.

Sapor had attacked the city, apparently, in the absence of Constantius, who had been called off to Pannonia to hold a conference with his brothers.