39.
The death of Constantine was followed by the division of the Roman world among his sons.

The vast empire with which Sapor had almost made up his mind to contend was partitioned out into three moderate-sized kingdoms.
In place of the late brave and experienced emperor, a raw youth, who had given no signs of superior ability, had the government of the Roman provinces of the East, of Thrace, Asia Minor, Syria, Mesopotamia, and Egypt.

Master of one third of the empire only, and of the least warlike portion, Constantius was a foe whom the Persian monarch might well despise, and whom he might expect to defeat without much difficulty.
Moreover, there was much in the circumstances of the time that seemed to promise success to the Persian arms in a struggle with Rome.

The removal of Constantme had been followed by an outburst of licentiousness and violence among the Roman soldiery in the capital; and throughout the East the army had cast off the restraints of discipline, and given indications of a turbulent and seditious spirit.

The condition of Armenia was also such as to encourage Sapor in his ambitious projects.
Tiridates, though a persecutor of the Christians in the early part of his reign, had been converted by Gregory the Illuminator, and had then enforced Christianity on his subjects by fire and sword.