But the death of that monarch in the early summer of A.D.337, on his way to the eastern frontier, dispelled the last chance of peace by relieving Sapor from the wholesome fear which had hitherto restrained his ambition.

The military fame of Constantine was great, and naturally inspired respect; his power was firmly fixed, and he was without competitor or rival.

By his removal the whole face of affairs was changed; and Sapor, who had almost brought himself to venture on a rupture with Rome during Constantine's life, no longer hesitated on receiving news of his death, but at once commenced hostilities.
It is probable that among the motives which determined the somewhat wavering conduct of Sapor at this juncture was a reasonable fear of the internal troubles which it seemed to be in the power of the Romans to excite among the Persians, if from friends they became enemies.

Having tested his own military capacity in his Arab wars, and formed an army on whose courage, endurance, and attachment he could rely, he was not afraid of measuring his strength with that of Rome in the open field; but he may well have dreaded the arts which the Imperial State was in the habit of employing, to supplement her military shortcomings, in wars with her neighbors.

There was now at the court of Constantinople a Persian refugee of such rank and importance that Constantine had, as it were, a pretender ready made to his hand, and could reckon on creating dissension among the Persians whenever he pleased, by simply proclaiming himself this person's ally and patron.