The Battle of the River Amnias was a battle fought in 89 BC between Pontus and Bithynia during the First Mithridatic War. The forces of Pontus were led by Archelaus and Neoptolemus, while the Bithynians were led by Nicomedes IV. Pontus was victorious. This battle was what would later be referred to as the start of the end of the Roman rule in the province of Asia.
The main force was all under the command of Mithridates who had an army of 250,000 compared to Nicomedes who only had 50,000 infantry and 6,000 cavalry. The Pontic forces at the Amnias River where this battle takes place were commanded by the brothers Neoptolemus and Archelaus, who had a force that according to Appian was still outnumbered by Nicomedes despite the large force commanded by Mithradates VI of Pontus.
The Battle 
The Battle of the Amnias River started around a rocky hill on the plains. Neoptolemus and Archelaus, generals under Mithiridates, sent troops forward to occupy the hill, but they were repelled by Nicomedes. Neoptolemus then led another attack on hill, but was repelled again. Nicomedes managed to force Neoptolemus' men to retreat, but before Nicomedes could finish off Neoptolemus, he was attacked on the flank by Archelaus.
Turning Point 
The turning point of the battle occurred when the Scythed chariots led by Archelaus dived into Nicomedes army. According to Appian the scythe chariots caused wounds of such a hideous nature that they caused fear and confusion among Nicomede’s army. An excerpt from the translated history that Appian wrote states "At the Battle of the Amnias River (89 BC), a victory that helped Mithridates VI wrest Asia Minor away from Rome, scythe-bearing chariots charged into Rome's Bithynian allies, "cutting some of them in two, and tearing others to pieces." So horrified was the army at the spectacle of men being cut in half while still breathing or their mangled bodies hanging in parts on the scythes that, "overcome rather by the hideousness of the spectacle than by the loss of the fight, fear took possession of their ranks"." This gave Neoptolemus time to regroup and attack what was now the rear of Nicomedes' armor. Even though Archelaus and Neoptolemus had the clear advantage the battle did not end until Neoptolemus basically lost his entire army.
The remaining survivors from Nicomedes' army fled into Paphlagonia, where they were able to join up with the Roman army under Manius Aquillius. Later this army would also be defeated, at the fortress of Protophachium. After that loss the Roman rule within the province of Asia was basically lost.
Coordinates: 40°25′26″N 34°08′10″E / 40.4238°N 34.136°E