Far more fraught with momentous effects than these new relations of the Romans to the Armenians, Iberians, Bosporans, and Nabataeans was the proximity into which through the occupation of Syria they were brought with the Parthian state. Complaisant as had been the demeanour of Roman diplomacy towards Phraates while the Pontic and Armenian states still subsisted, willingly as both Lucullus and Pompeius had then conceded to him the possession of the regions beyond the Euphrates[19], the new neighbour now sternly took up his position by the side of the Arsacids; and Phraates, if the royal art of forgetting his own faults allowed him, might well recall now the warning words of Mithradates that the Parthian by his alliance with the Occidentals against the kingdoms of kindred race paved the way first for their destruction and then for his own. Romans and Parthians in league had brought Armenia to ruin; when it was overthrown, Rome true to her old policy now reversed the parts and favoured the humbled foe at the expense of the powerful ally. The singular preference, which the father Tigranes experienced from Pompeius as contrasted with his son the ally and son-in-law of the Parthian king, was already part of this policy; it was a direct offence, when soon afterwards by the orders of Pompeius the younger Tigranes and his family were arrested and were not released even on Phraates interceding with the friendly general for his daughter and his son-in-law.

But Pompeius paused not here. The province of Corduene, to which both Phraates and Tigranes laid claim, was at the command of Pompeius occupied by Roman troops for the latter, and the Parthians who were found in possession were driven beyond the frontier and pursued even as far as Arbela in Adiabene, without the government of Ctesiphon having even been previously heard (689).

Far the most suspicious circumstance however was, that the Romans seemed not at all inclined to respect the boundary of the Euphrates fixed by treaty. On several occasions Roman divisions destined from Armenia for Syria marched across Mesopotamia; the Arab emir Abgarus of Osrhoene was received under singularly favourable conditions into Roman protection; nay, Oruros, situated in Upper Mesopotamia somewhere between Nisibis and the Tigris 220 miles eastward from the Commagenian passage of the Euphrates, was designated as the eastern limit of the Roman dominion - presumably their indirect dominion, inasmuch as the larger and more fertile northern half of Mesopotamia had been assigned by the Romans in like manner with Corduene to the Armenian empire.

The boundary between Romans and Parthians thus became the great Syro-Mesopotamian desert instead of the Euphrates; and this too seemed only provisional. To the Parthian envoys, who came to insist on the maintenance of the agreements - which certainly, as it would seem, were only concluded orally - respecting the Euphrates boundary, Pompeius gave the ambiguous reply that the territory of Rome extended as far as her rights. The remarkable intercourse between the Roman commander-in-chief and the Parthian satraps of the region of Media and even of the distant province Elymais (between Susiana, Media, and Persia, in the modern Luristan) seemed a commentary on this speech[20]. The viceroys of this latter mountainous, warlike, and remote land had always exerted themselves to acquire a position independent of the great-king; it was the more offensive and menacing to the Parthian government, when Pompeius accepted the proffered homage of this dynast.

Not less significant was the fact that the title of "king of kings" which had been hitherto conceded to the Parthian king by the Romans in official intercourse, was now all at once exchanged by them for the simple title of king. This was even more a threat than a violation of etiquette. Since Rome had entered on the heritage of the Seleucids, it seemed almost as if the Romans had a mind to revert at a convenient moment to those old times, when all Iran and Turan were ruled from Antioch, and there was as yet no Parthian empire but merely a Parthian satrapy. The court of Ctesiphon would thus have had reason enough for going to war with Rome; it seemed the prelude to its doing so, when in 690 it declared war on Armenia on account of the question of the frontier. But Phraates had not the courage to come to an open rupture with the Romans at a time when the dreaded general with his strong army was on the borders of the Parthian empire. When Pompeius sent commissioners to settle amicably the dispute between Parthia and Armenia, Phraates yielded to the Roman mediation forced upon him and acquiesced in their award, which assigned to the Armenians Corduene and northern Mesopotamia. Soon afterwards his daughter with her son and her husband adorned the triumph of the Roman general. Even the Parthians trembled before the superior power of Rome; and, if they had not, like the inhabitants of Pontus and Armenia, succumbed to the Roman arms, the reason seemed only to be that they had not ventured to stand the conflict.

There still devolved on the general the duty of regulating the internal relations of the newly-acquired provinces and of removing as far as possible the traces of a thirteen years' desolating war.

The work of organization begun in Asia Minor by Lucullus and the commission associated with him, and in Crete by Metellus, received its final conclusion from Pompeius. The former province of Asia, which embraced Mysia, Lydia, Phrygia, and Caria, was converted from a frontier province into a central one. The newly-erected provinces were, that of Bithynia and Pontus, which was formed out of the whole former kingdom of Nicomedes and the western half of the former Pontic state as far as and beyond the Halys; that of Cilicia, which indeed was older, but was now for the first time enlarged and organized in a manner befitting its name, and comprehended also Pamphylia and Isauria; that of Syria, and that of Crete. Much was no doubt wanting to render that mass of countries capable of being regarded as the territorial possession of Rome in the modern sense of the term. The form and order of the government remained substantially as they were; only the Roman community came in place of the former monarchs. Those Asiatic provinces consisted as formerly of a motley mixture of domanial possessions, urban territories de facto or de jure autonomous, lordships pertaining to princes and priests, and kingdoms, all of which were as regards internal administration more or less left to themselves, and in other respects were dependent, sometimes in milder sometimes in stricter forms, on the Roman government and its proconsuls very much as formerly on the great-king and his satraps.

The first place, in rank at least, among the dependent dynasts was held by the king of Cappadocia, whose territory Lucullus had already enlarged by investing him with the province of Melitene (about Malatia) as far as the Euphrates, and to whom Pompeius farther granted on the western frontier some districts taken off Cilicia from Castabala as far as Derbe near Iconium, and on the eastern frontier the province of Sophene situated on the left bank of the Euphrates opposite Melitene and at first destined for the Armenian prince Tigranes; so that the most important passage of the Euphrates thus came wholly into the power of the Cappadocian prince. The small province of Commagene between Syria and Cappadocia with its capital Samosata (Samsat) remained a dependent kingdom in the hands of the already-named Seleucid Antiochus[21]; to him too were assigned the important fortress of Seleucia (near Biradjik) commanding the more southern passage of the Euphrates, and the adjoining tracts on the left bank of that river; and thus care was taken that the two chief passages of the Euphrates with a corresponding territory on the eastern bank were left in the hands of two dynasts wholly dependent on Rome. Alongside of the kings of Cappadocia and Commagene, and in real power far superior to them, the new king Deiotarus ruled in Asia Minor. One of the tetrarchs of the Celtic stock of the Tolistobogii settled round Pessinus, and summoned by Lucullus and Pompeius to render military service with the other small Roman clients, Deiotarus had in these campaigns so brilliantly proved his trustworthiness and his energy as contrasted with all the indolent Orientals that the Roman generals conferred upon him, in addition to his Galatian heritage and his possessions in the rich country between Amisus and the mouth of the Halys, the eastern half of the former Pontic empire with the maritime towns of Pharnacia and Trapezus and the Pontic Armenia as far as the frontier of Colchis and the Greater Armenia, to form the kingdom of Lesser Armenia. Soon afterwards he increased his already considerable territory by the country of the Celtic Trocmi, whose tetrarch he dispossessed. Thus the petty feudatory became one of the most powerful dynasts of Asia Minor, to whom might be entrusted the guardianship of an important part of the frontier of the empire.