(ii.) The Tigris is said by Patricius and Festus to have been made the boundary of the two empires. Gibbon here boldly substitutes the Western Khabour and maintains that "the Roman frontier traversed, but never followed, the course of the Tigris." He appears not to be able to understand how the Tigris could be the frontier, when five provinces across the Tigris were Roman. But the intention of the article probably was, first, to mark the complete cession to Rome of Eastern as well as Western Mesopotamia, and, secondly, to establish the Tigris as the line separating the empires below the point down to which the Romans held both banks. Cordyene may not have touch the Tigris at all, or may have touched it only about the 37th parallel. From this point southwards, as far as Mosul, or Nimrud, or possibly Kileh Sherghat, the Tigris was probably now recognized as the dividing line between the empires. By the letter of the treaty the whole Euphrates valley might indeed have been claimed by Rome; but practically she did not push her occupation of Mesopotamia below Circeshim. The real frontier from this point was the Mesopotamian desert, which extends from Kerkesiyeh to Nimrud, a distance of 150 miles. Above this it was the Tigris, as far probably as Feshapoor; after which it followed the line, whatever it was, which divided Oordyene from Assyria and Media.
(iii.) The extension of Armenia to the fortress of Zintha, in Media, seems to have imported much more than would at first sight appear from the words. Gibbon interprets it as implying the cession of all Media Atropatene, which certainly appears a little later to be in the possession of the Armenian monarch, Tiridates. A large addition to the Armenian territory out of the Median is doubtless intended; but it is quite impossible to determine definitely the extent or exact character of the cession.
(iv.) The fourth article of the treaty is sufficiently intelligible. So long as Armenia had been a fief of the Persian empire, it naturally belonged to Persia to exercise influence over the neighboring Iberia, which corresponded closely to the modern Georgia, intervening between Armenia and the Caucasus. Now, when Armenia had become a dependency of Rome, the protectorate hitherto exercised by the Sassanian princes passed naturally to the Caesars; and with the protectorate was bound up the right of granting investiture to the kingdom, whereby the protecting power was secured against the establishment on the throne of an unfriendly person. Iberia was not herself a state of much strength; but her power of opening or shutting the passes of the Caucasus gave her considerable importance, since by the admission of the Tatar hordes, which were always ready to pour in from the plains of the North, she could suddenly change the whole face of affairs in North-Western Asia, and inflict a terrible revenge on any enemy that had provoked her. It is true that she might also bring suffering on her friends, or even on herself, for the hordes, once admitted, were apt to make little distinction between friend and foe; but prudential considerations did not always prevail over the promptings of passion, and there had been occasions when, in spite of them, the gates had been thrown open and the barbarians invited to enter. It was well for Rome to have it in her power to check this peril. Her own strength and the tranquillity of her eastern provinces were confirmed and secured by the right which she (practically) obtained of nominating the Iberian monarchs.