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
Invasion of Pontus - Retreat of Mithradates
So soon as the season allowed, the Roman troops crossed the frontier of Pontus. There they were opposed by king Mithradates with 30,000 infantry and 3000 cavalry. Left in the lurch by his allies and attacked by Rome with reinforced power and energy, he made an attempt to procure peace; but he would hear nothing of the unconditional submission which Pompeius demanded--what worse could the most unsuccessful campaign bring to him?
That he might not expose his army, mostly archers and horsemen, to the formidable shock of the Roman infantry of the line, he slowly retired before the enemy, and compelled the Romans to follow him in his various cross-marches; making a stand at the same time, wherever there was opportunity, with his superior cavalry against that of the enemy, and occasioning no small hardship to the Romans by impeding their supplies.
At length Pompeius in his impatience desisted from following the Pontic army, and, letting the king alone, proceeded to subdue the land; he marched to the upper Euphrates, crossed it, and entered the eastern provinces of the Pontic empire. But Mithradates followed along the left bank of the Euphrates, and when he had arrived in the Anaitic or Acilisenian province, he intercepted the route of the Romans at the castle of Dasteira, which was strong and well provided with water, and from which with his light troops he commanded the plain.
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Reference address : https://www.ellopos.net/elpenor/rome/5-04-pompeius-east.asp?pg=11