Res Gestae

But the Persians in the East, rather by thieving and robbery than (as their former manner was) in set battles, kept driving off booty of men and animals; sometimes they got away with their loot, being unexpected; often they lost it, over-marched by the great number of our soldiers; occasionally they were not allowed to see anything at all which could be carried off.

None the less, Musonianus, the praetorian prefect, a man (as I have said before) gifted with many excellent accomplishments, but corrupt and easy to turn from the truth by a bribe, inquired into the designs of the Persians through emissaries of his who were adepts in deceit and incrimination; and he took into his counsels on this subject Cassianus, duke of Mesopotamia, who had been toughened by various campaigns and dangers.

When the two had certain knowledge from the unanimous reports of their scouts that Sapor, on the remotest frontiers of his realm, was with difficulty and with great bloodshed of his troops driving back hostile tribesmen, they made trial of Tamsapor, the commander nearest to our territory, in secret interviews through obscure soldiers, their idea being that, if chance gave an opportunity, he should by letter advise the king finally to make peace with the Roman emperor, in order that by so doing he might be secure on his whole western frontier and could rush upon his persistent enemies.

Tamsapor consented and relying on this information, reported to the king that Constantius, being involved in very serious wars, entreated and begged for peace.