With Caesar preoccupied in Egypt and the Roman Republic in the midst of a civil war, Pharnaces saw an opportunity to expand his Kingdom of the Bosphorus into his father's old Pontic empire.
Calvinus advanced toward Pontus in order to strengthen his forces with military settlers hastily recruited from Rome's Pontic colonies.
Pharnaces tried to delay Calvinus by diplomatic means but, failing in this, retired to the vicinity of Nicopolis in Armenia Parva.
With a large segment of his line now gone, Calvinus could not assault the enemy positions with any hope of victory and had little options but to retreat.
The steadiness of the 36th legion saving him from complete annihilation; they formed a fighting circle called Orbis and fought their way to some nearby hills.
Although the 36th Legion escaped with light losses, just 250 casualties, Calvinus had lost nearly two thirds of his army by the time he had fully disengaged.