The battle of Protopachium, 89 B.C., was the second of two victories won by the armies of Mithridates VI of Pontus that at least temporarily destroyed Roman authority in their province of Asia at the start of the First Mithridatic War. At the start of the war the Romans had four armies on the borders of Mithridates's kingdom of Pontus and his puppet state of Cappadocia. The northernmost of these armies, under the command of Manius Aquillius, was posted on the route that Mithridates was likely to take if he decided to invade Bithynia, while a second army, under Nicomedes IV of Bithynia, was sent east to take up a position in eastern Paphlagonia. Aquillius's army was largely made up of troops from Bithynia, exiles from Cappadocia, Paphlagonians and Galatians, and contained very few Roman troops.
Nicomedes reached the River Amnias, where he was defeated by part of the Pontic army under the command of the brothers Neoptolemus and Archelaus. He then retreated back to join Aquillius, before moving again to join C. Cassius, the governor of Asia, who had been posted further south, on the border between Bithynia and Asia.
After his generals had defeated Nicomedes, Mithridates led his main army into eastern Bithynia. According to Appian, Mithridates had 250,000 infantry, 40,000 cavalry and 130 scythed war chariots. This figure can probably be halved, but even so he still outnumbered Aquillius, who Appian gives 40,000 men. After the departure of Nicomedes, Aquillius seems to have decided to retreat back to the river Sangarius.
Part of Mithridates's army, under the command of Neoptolemus and Nemanes of Armenia, caught up with Aquillius's army close to the fortress of Protopachium, in eastern Bithynia, and forced a battle. Appian states that Aquillius lost 10,000 of his 44,000 men before he was able to escape, eventually reaching temporary safety at Pergamum, the capital of the Roman province of Asia.
Although two Roman armies remained intact, neither of their commanders had any confidence in their fighting ability, and in a short time Mithridates had managed to establish control over almost the entire Roman province. Aquillius met with a particularly gruesome fate. He attempted to reach safety on Lesbos, but was handed over to Mithridates. After keeping him on display for a period, Mithridates had him killed by pouring molten gold down his throat.