The incongruity between the national and the political connections of Armenia forms an essential element in the conflict--prolonged through the whole imperial period--with its eastern neighbour. It was discerned doubtless on the Roman side that annexation beyond the Euphrates was an encroachment on the family-domain of Oriental nationality, and was not any increase proper of power for Rome. But the ground or, if the phrase be preferred, the excuse for the continuance of such encroachment lay in the fact that the subsistence side by side of great states with equal rights was incompatible with the system of Roman policy, we may even say with the policy of antiquity in general. The Roman empire knew as limit, in the strict sense, only the sea or a land-district unarmed. To the weaker but yet warlike commonwealth of the Parthians the Romans always grudged a position of power, and took away from it what these in their turn could not forego; and therefore the relation between Rome and Iran through the whole imperial period was one of perpetual feud, interrupted only by armistices, concerning the left bank of the Euphrates.
[Sidenote: The Parthians during the civil wars.]
In the treaties concluded with the Parthians by Lucullus (iv. 71) { iv. 67.} and Pompeius (iv. 127) { iv. 122.} the Euphrates was recognised as the boundary, and so Mesopotamia was ceded to them. But this did not prevent the Romans from receiving the rulers of Edessa among their clients, and from laying claim to a great part of northern Mesopotamia at least for their indirect rule, apparently by extending the limits of Armenia towards the south (iv. 146) { iv. 140.}. On that account, after some delay, the Parthian government began the war against the Romans, in the form of declaring it against the Armenians. The answer to this was the campaign of Crassus, and, after the defeat at Carrhae (iv. 351 f.) { iv. 335 f.}, the bringing back of Armenia under Parthian power; we may add, the resumption of their claims on the western half of the Seleucid state, the carrying out of which, it is true, proved at that time unsuccessful (iv. 356) { iv. 339.}. During the whole twenty years of civil war, in which the Roman republic perished and ultimately the principate was established, the state of war between the Romans and Parthians continued, and not seldom the two struggles became intermixed. Pompeius had, before the decisive battle, attempted to gain king Orodes as ally; but, when the latter demanded the cession of Syria, Pompeius could not prevail on himself to deliver up the province which he had personally made Roman. After the catastrophe he had nevertheless resolved to do so; but accidents directed his flight not to Syria, but to Egypt, where he met his end (iv. 446) { iv. 424.}. The Parthians appeared on the point of once more breaking into Syria; and the later leaders of the republicans did not disdain the aid of the public foe. Even in Caesar’s lifetime Caecilius Bassus, when he raised the banner of revolt in Syria, had at once called in the Parthians. They had followed this call; Pacorus, the son of Orodes, had defeated Caesar’s lieutenant and liberated the troops of Bassus besieged by him in Apamea (709) { 44 B.C.}. For this reason, as well as in order to take revenge for Carrhae, Caesar had resolved to go in the next spring personally to Syria and to cross the Euphrates; but his death prevented the execution of this plan. When Cassius thereupon took arms in Syria, he entered into relations with the Parthian king; and in the decisive battle at Philippi (712) { 42 B.C.} Parthian mounted archers joined in fighting for the freedom of Rome. When the republicans succumbed, the great-king, in the first instance, maintained a quiet attitude; and Antonius, while designing probably to execute the plans of the dictator, had at first enough to do with the settlement of the East. The collision could not fail to take place; the assailant this time was the Parthian king.