Evenerat . . . quasi fatali constellatione . . . ut Constantium dimicantem cum Persis fortuna semper sequeretur afflictior.--Amm. Marc. xx. 9, ad fin.
It seems to have been soon after the close of Sapor's first war with Constantius that events took place in Armenia which once more replaced that country under Roman influence. Arsaces, the son of Tiranus, had been, as we have seen, established as monarch, by Sapor, in the year A.D. 341, under the notion that, in return for the favor shown him, he would administer Armenia in the Persian interest. But gratitude is an unsafe basis for the friendships of monarchs. Arsaces, after a time, began to chafe against the obligations under which Sapor had laid him, and to wish, by taking independent action, to show himself a real king, and not a mere feudatory. He was also, perhaps, tired of aiding Sapor in his Roman war, and may have found that he suffered more than he gained by having Rome for an enemy. At any rate, in the interval between A.D. 351 and 359, probably while Sapor was engaged in the far East, Arsaces sent envoys to Constantinople with a request to Constantius that he would give him in marriage a member of the Imperial house. Constantius was charmed with the application made to him, and at once accepted the proposal. He selected for the proffered honor a certain Olympias, the daughter of Ablabius, a Praetorian prefect, and lately the betrothed bride of his own brother, Constans; and sent her to Armenia, where Arsaces welcomed her, and made her (as it would seem) his chief wife, provoking thereby the jealousy and aversion of his previous sultana, a native Armenian, named Pharandzem. The engagement thus entered into led on, naturally, to the conclusion of a formal alliance between Rome and Armenia--an alliance which Sapor made fruitless efforts to disturb, and which continued unimpaired down to the time A.D. 359 when hostilities once more broke out between Rome and Persia.
Of Sapor's Eastern wars we have no detailed account. They seem to have occupied him from A.D. 350 to A.D. 357, and to have been, on the whole, successful. They were certainly terminated by a peace in the last-named year--a peace of which it must have been a condition that his late enemies should lend him aid in the struggle which he was about to renew with Rome. Who these enemies exactly were, and what exact region they inhabited, is doubtful. They comprised certainly the Chionites and Gelani, probably the Euseni and the Vertse. The Chionites are thought to have been Hiongnu or Huns; and the Euseni are probably the Usiun, who, as early as B.C. 200, are found among the nomadic hordes pressing towards the Oxus. The Vertse are wholly unknown. The Gelani should, by their name, be the inhabitants of Ghilan, or the coast tract south-west of the Caspian; but this locality seems too remote from the probable seats of the Chionites and Euseni to be the one intended. The general scene of the wars was undoubtedly east of the Caspian, either in the Oxus region, or still further eastward, on the confines of India and Scythia. The result of the wars, though not a conquest, was an extension of Persian influence and power. Troublesome enemies were converted into friends and allies. The loss of a predominating influence over Armenia was thus compensated, or more than compensated, within a few years, by a gain of a similar kind in another quarter.