The complete triumph of Orodes over Mithridates, and his full establishment in his kingdom, cannot be placed earlier than B.C. 56, and most probably fell in B.C. 55.  In this latter year Crassus obtained the consulship at Rome, and, being appointed at the same time to the command of the East, made no secret of his intention to march the Roman legions across the Euphrates, and engage in hostilities with the great Parthian kingdom.  According to some writers, his views extended even further.  He spoke of the wars which Lucullus had waged against Tigranes and Pompey against Mithridates of Pontus as mere child’s play, and announced his intention of carrying the Roman arms to Bactria, India, and the Eastern Ocean.  The Parthian king was thus warned betimes of the impending danger, and enabled to make all such preparations against it as he deemed necessary.  More than a year elapsed between the assignment to Crassus of Syria as his province, and his first overt act of hostility against Orodes.

It cannot be doubted that this breathing-time was well spent by the Parthian monarch.  Besides forming his general plan of campaign at his leisure, and collecting, arming, and exercising his native forces, he was enabled to gain over certain chiefs upon his borders, who had hitherto held a semi-dependent position, and might have been expected to welcome the Romans.  One of these, Abgarus, prince of Osrhoene, or the tract east of the Euphrates about the city of Edessa, had been received into the Roman alliance by Pompey, but, with the fickleness common among Orientals, he now readily changed sides, and undertook to play a double part for the advantage of the Parthians.  Another, Alchaudonius, an Arab sheikh of these parts, had made his submission to Rome even earlier; but having become convinced that Parthia was the stronger power of the two, he also went over to Orodes.  The importance of these adhesions would depend greatly on the line of march which Crassus might determine to follow in making his attack.  Three plans were open to him.  He might either throw himself on the support of Artavasdes, the Armenian monarch, who had recently succeeded his father Tigranes, and entering Armenia, take the safe but circuitous route through the mountains into Adiabene, and so by the left bank of the Tigris to Ctesiphon; or he might, like the younger Cyrus, follow the course of the Euphrates to the latitude of Seleucia, and then cross the narrow tract of plain which there separates the two rivers; or, finally, he might attempt the shortest but most dangerous line across the Belik and Khabour, and directly through the Mesopotamian desert.  If the Armenian route were preferred, neither Abgarus nor Alchaudonius would be able to do the Parthians much service; but if Crassus resolved on following either of the others, their alliance could not but be most valuable.