The battle of the Halys River was the only major engagement during the short Second Mithridatic War (83-82 B.C.) and was one of the few defeats suffered by a Roman army during the three wars against Mithridates VI of Pontus.
At the end of the First Mithridatic War Sulla had left Lucius Licinius Murena to settle the affairs of Asia Minor, giving him the two legions that had formerly been commanded by Sulla's opponent Fimbria. While Sulla wanted to preserve the peace in Asia Minor, Murena seems to have wanted to provoke a war to earn himself a triumph, and in 83 B.C. he found the excuse he wanted. Mithridates VI had been preparing for an expedition across the Black Sea to deal with a rebellion in his northern territories. Murena professed to see this army as a threat to Rome, a view that was supported by defection of the former Pontic general Archelaus. Murena carried out two raids into Pontic territory without provoking a military response, and then in 82 B.C., despite having received a public order from the Senate not to continue the war, launched a third raid.
This time Mithridates reacted. An army under the Cappadocian noble and Pontic general Gordius was sent to oppose Murena. The two armies came face to face across a river, probably the Halys. Gordius then waited until Mithridates arrived in person with the main army.
The combined Pontic army attacked the Roman positions on the river banks, and eventually forced their way across the river. Murena retreated onto a strong position on a nearby hill, but was eventually forced to abandon the hilltop, and retreat across the mountains to Phrygia.
In the aftermath of the battle Mithridates drove the Romans out of Cappadocia, and was on the verge of occupying the kingdom, when the war was ended by the arrival of a message from Sulla. Murena was ordered to stop the fighting, and to arrange for a reconciliation between Mithridates and Ariobarzanes. The war was ended by the betrothal of Ariobarzanes and Mithridates's four year old daughter, and by the transfer of a strip of Cappadocia to Pontic control.