The Battle of Protopachium was fought in 89 BC at the start of the First Mithridatic War, between the Roman Republic and the Pontic Empire.

[1] Aquillius found Nicomedes retreating south with Gaius Cassius (the proconsul of Asia) and decided to patrol eastern Bithynia, where Mithridates was likely to go.

When Aquillius saw that he was severely outnumbered, he retreated to river Sakarya, which part of Mithridates' army (under the command of Neoptolemus and Nemanes) eventually caught up with them near the fortress of Protopachium in eastern Bithynia.

Apart from being outnumbered, an additional reason Aquillius lost was that his army was largely consisted troops from Bithynia, exiles from Cappadocia, Paphlagonians, and Galatians.

When Aquillius reached Lesbos, he was handed over to Mithridates by the people of Mytilene, and had molten gold poured down his throat (a similar fate is said to have happened with Marcus Licinius Crassus – 'richest person in Rome').