The Battle of Orchomenus was fought in 85 BC between Rome and the forces of Mithridates VI of Pontus. The Roman army was led by Lucius Cornelius Sulla, while Mithridates' army was led by Archelaus. The Roman force was victorious, and Archelaus later defected to Rome. Information on the battle is included in Plutarch's Life of Sulla, chapters 20-21.
After his victory over Archelaus at Chaeronea, Sulla set out for Thessaly to meet the consul Flaccus coming from Italy (although Sulla was unaware he had been sent to attack him, not to join with him). On the way, he heard reports that Dorylaeus had landed at Chalcis with a sizeable fleet transporting eighty thousand of Mithridates' best troops to reinforce Archelaus. Dorylaeus wanted to tempt Sulla to fight as soon as possible, and Sulla cooperated by abruptly turning around to meet this new threat. After a skirmish with Sulla's troops, Dorylaeus began to rethink the idea of giving battle and instead promoted a strategy to wear the enemy down. On the other hand, Archelaus' confidence was raised by the flat terrain around their camp at Orchomenus, which favored their superior cavalry.
While Archelaus let his men relax after taking their positions, Sulla set his men to work building trenches and ditches which he hoped would cut Archelaus' cavalry off from the plains and move the fighting to more boggy areas. Archelaus recognized Sulla's strategy, and launched several attacks on the legionnaires digging the trenches and ditches . In one of these, Archelaus' stepson Diogenese distinguished himself in a valiant attack where he died gloriously. In Archelaus' final attack, Sulla routed his troops and carried his camp. Plutarch says that so many men died that the marshes ran with blood, and almost two hundred years later weapons were still found sticking out of the marshes. After the battle, he destroyed three Boeotian towns: Anthedon, Darymna, and Halae. Later, upon meeting fishermen from Halae who gave him fish, Sulla told them he was surprised there were any of them left, but let them go and told them not to worry. As a result of this incident, the people of Halae were inspired to repopulate their town.
While Sulla was away fighting Mithridates, Rome was suffering from civil disorder at the hands of the two consuls of 85 BC, Cinna and Carbo, prompting eminent members of society to flee to Sulla's camp, including his wife Metella and their children. Sulla tried to use his victory at the Battle of Orchomenus to bring about peace with Mithridates so that he could return home, and though Sulla's peace terms were not immediately accepted, Archelaus eventually managed to broker a peace between Sulla and Mithridates. After Fimbria's troops defected to Sulla (originally the troops of Flaccus, who Fimbria had led a revolt against), Fimbria committed suicide and Sulla's was able to wrap up his affairs in Greece and Asia Minor, and return to Italy.