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
Tigranes Breaks with Mithradates - Mithradates Crosses the Phasis
This hope likewise was vain; the alliance, on the faith of which Mithradates took the route for Armenia, already by that time existed no longer. During the conflicts between Mithradates and Pompeius just narrated, the king of the Parthians, yielding to the urgency of the Romans and above all of the exiled Armenian prince, had invaded the kingdom of Tigranes by force of arms, and had compelled him to withdraw into the inaccessible mountains. The invading army began even the siege of the capital Artaxata; but, on its becoming protracted, king Phraates took his departure with the greater portion of his troops; whereupon Tigranes overpowered the Parthian corps left behind and the Armenian emigrants led by his son, and re-established his dominion throughout the kingdom.
Naturally, however, the king was under such circumstances little inclined to fight with the freshly-victorious Romans, and least of all to sacrifice himself for Mithradates; whom he trusted less than ever, since information had reached him that his rebellious son intended to betake himself to his grandfather. So he entered into negotiations with the Romans for a separate peace; but he did not wait for the conclusion of the treaty to break off the alliance which linked him to Mithradates.
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Reference address : http://www.ellopos.net/elpenor/rome/5-04-pompeius-east.asp?pg=14