Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
War Preparations of Pompeius - Alliance with the Parthians - Variance between Mithradates and Tigranes
The few months which still remained before the commencement of the campaign in Asia Minor, were employed by the new commander- in-chief with strenuous activity in diplomatic and military preparations. Envoys were sent to Mithradates, rather to reconnoitre than to attempt a serious mediation. There was a hope at the Pontic court that Phraates king of the Parthians would be induced by the recent considerable successes which the allies had achieved over Rome to enter into the Pontic-Armenian alliance. To counteract this, Roman envoys proceeded to the court of Ctesiphon; and the internal troubles, which distracted the Armenian ruling house, came to their aid.
A son of the great-king Tigranes, bearing the same name had rebelled against his father, either because he was unwilling to wait for the death of the old man, or because his father's suspicion, which had already cost several of his brothers their lives, led him to discern his only chance of safety in open insurrection. Vanquished by his father, he had taken refuge with a number of Armenians of rank at the court of the Arsacid, and intrigued against his father there. It was partly due to his exertions, that Phraates preferred to take the reward which was offered to him by both sides for his accession--the secured possession of Mesopotamia--from the hand of the Romans, renewed with Pompeius the agreement concluded with Lucullus respecting the boundary of the Euphrates,(4) and even consented to operate in concert with the Romans against Armenia.
4. Cf. V. II. Renewal of the War
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Reference address : https://www.ellopos.net/elpenor/rome/5-04-pompeius-east.asp?pg=8