The battle of the Amnias River (89 B.C.) was the first battle of the First Mithridatic War, and was the first of a series of victories in which the armies of Mithridates VI conquered the Roman province of Asia. At the outbreak of the war the Romans and their allies had four armies, which if combined would probably still have been smaller than the forces available to Mithridates, but these four armies were spread out along the western borders of Pontus and Cappadocia.
The largest of these four armies was that of Nicomedes IV of Bithynia, who was said to have had 50,000 infantry and 6,000 cavalry. With this army Nicomedes was to advance into eastern Paphlagonia to form the northern flank of the Roman defensive line, but the advance met with disaster.
Nicomedes began his advance at Bithynium, in eastern Bithynia. He advanced into Paphlagonia, and reached the river Amnias before running into part of the Pontic army on the wide plain that bordered the river Amnias.
The main Pontic army was under the direct command of Mithridates, but the early battles were all won by his generals. The Pontic force at the Amnias River was commanded by the brothers Neoptolemus and Archelaus, who had command of a force of light infantry, cavalry from Armenia Minor (under the command of Mithridates's son Arcathias), and some scythed chariots. Appian, who is our main source for this battle, and for the size of the opposing armies, gives Mithridates a total of 250,000 infantry, but also states that Neoptolemus and Archelaus were outnumbered at the Amnias River.
The battle developed around a rocky hill on the plains. Neoptolemus and Archelaus sent troops forward to occupy the hill, before being pushed off by Nicomedes. Neoptolemus then led a second attack on the hill, and once again Nicomedes responded in strength. Neoptolemus's men were put to flight, but before Nicomedes could take advantage of this he was attacked from the right by Archelaus. Nicomedes was forced to stop the pursuit, and turn his army to face the new threat.
The turning point of the battle came when the scythed chariots made their attack. According to Appian the hideous nature of the wounds they caused created fear and confusion in the Bithynian army. This gave Neoptolemus time to rally his men and launch an attack on what was now the Bithynian rear. This final stage of the battle is said to have lasted for some time, and only ended after Nicomedes had lost most of his army. The survivors fled into Paphlagonia, where they joined up with the Roman army under Manius Aquillius. When this army was also defeated, at the fortress of Protophachium, the entire edifice of Roman rule in the province of Asia collapsed.