In the geographical aspect of this long struggle three special periods need to be pointed out. ♦Conquests of Trajan. A.D. 114-117.♦ The first is that of the momentary conquests of Trajan. Under him _Armenia_, hitherto a vassal kingdom of Rome, was incorporated as a Roman province. _Albania_ and _Iberia_ took its place as the frontier vassal states. Beyond the Euphrates, even beyond the Tigris, the Roman dominion took in _Mesopotamia_, _Atropatênê_, and _Babylonia_. The Parthian capital of Ktesiphôn and the outlying Greek free city of Seleukeia were included within the boundaries of an Empire which for a moment touched the Caspian and the Persian Gulf. Rome, as the champion of the West, seemed to have triumphed for ever over her Eastern rival, when the Parthian kingdom was thus shorn of the border lands of the two worlds, and when its king was forced to become a Roman vassal for the dominions that were left to him. But this vast extension of the Roman power was strictly only for a moment. ♦Conquests of Trajan surrendered by Hadrian. A.D. 117.♦ What Trajan had conquered Hadrian at once gave back; the Empire was again bounded by the Euphrates, and Armenia was again left to form matter of dispute between its Eastern and its Western claimant. ♦Conquests of Marcus. A.D. 162-166.♦ The second stage begins when, under Marcus, the Roman frontier again began to advance. ♦Of Severus. A.D. 197-202.♦ Between the Euphrates and the Tigris _Osrhoênê_ became a Roman dependency: under the house of Severus it became a Roman province; and the fortress of _Nisibis_, so famous in later wars, was planted as the Eastern outpost of Rome against the Parthian. Ten years later the Parthian power was no more; but, as seen with Western eyes, the revived monarchy of Persia had simply stepped into its place. The wars of Alexander Severus, the captivity of Valerian, the wasting march of Sapor through the Roman provinces, left no trace on the map. ♦Conquests under Diocletian. A.D. 297.♦ But under the mighty rule of Diocletian the glories of Trajan were renewed. Mesopotamia again became Roman; five provinces beyond the Tigris were added to the Empire; Armenia, again the vassal of Rome, was enlarged at the expense of Persia, and Iberia was once more a Roman dependency. In the third stage the Roman frontier again went back. The wars of the second Sapor did little but deprive Rome of two Mesopotamian fortresses. ♦Surrender of provinces by Jovian. A.D. 363.♦ But after the fall of Julian the lands beyond the Tigris were given back to Persia; even Nisibis was yielded, and the Persian frontier again reached the Euphrates. ♦Division of Armenia. 387. | The Hundred Years’ Peace. 421.♦ Armenia was now tossed to and fro, conquered and reconquered, till the kingdom was divided between the vassals of the two Empires, a division which was again confirmed by the hundred years’ peace between Rome and Persia. This was the state of the Eastern frontier of Rome at the time