After defeating Pompey the Great and the optimates at Pharsalus, Julius Caesar went to Asia Minor and then to Egypt. In Asia province he left Calvinus in command with an army including the XXXVI Legion, made up mainly of veterans from Pompey's disbanded legions. With Caesar preoccupied in Egypt and the Roman republic in the midst of civil strife, Pharnaces saw an opportunity to expand his Kingdom of the Bosphorus into his father's old Pontic empire. In 48 BC he invaded Cappadocia, Bithynia, and Armenia Parva. Calvinus concentrated his forces at Comana. These forces consisted of the veteran Roman XXXVI Legion, and two recently levied green local legions recruited in Armenia Parva, which were armed, trained, and organized in the Roman style by King Deiotarus of Galatia. He also had local auxiliary skirmishers and cavalry from Cilicia. Despite being outnumbered by Pharnaces, 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. Calvinus brought his army to within seven miles of Nicopolis and, avoiding an ambush set by Pharnaces, deployed his army. Pharnaces now retired to the city and awaited a further Roman advance.
Battle
Calvinus advanced to find Pharnaces' heavy infantry formed in deep ranks between two trenches, fronted by his skirmishers and flanked by numerous cavalry beyond the trenches. The Romans deployed the XXXVIth legion on the right wing, the force of former legionaries recruited from the Pontic colonies on the left and the recently levied legions in the center. The Roman auxiliaries formed the advance guard and what little cavalry the Romans had were put on the flanks. As Pharnaces outnumbered the Romans, Calvinus spread his army thin in order to match Pharances' deployment and avoid being outflanked. The battle began poorly for Calvinus as his levied Pontic troops fled soon after the onset of fighting. With a large segment of his line now gone, Calvinus could not assault the enemy positions with any hope of victory and had little option but to retreat. The steadiness of the XXXVI legion saving him from complete annihilation, he oversaw a fighting withdrawal. Although the XXXVI Legion escaped with light losses, Calvinus had lost nearly two thirds of his army by the time he had fully disengaged.
Aftermath
A subsequent rebellion in his rear prevented Pharnaces from capitalizing on his victory, forcing him to return to deal with it. By the time the rebellion was quelled, Caesar had arrived to rectify the situation and then decisively defeated Pharnaces at the Battle of Zela.
