The battle of Orchomenus of 86 B.C. was the second of two great Roman victories that ended the Pontic invasion of Greece during the First Mithridatic War. A large Pontic army under the general Archelaus had reached Greece in 88 B.C., and found allies in Athens and throughout southern Greece. The Romans had responded by sending five legions under Lucius Sulla to Greece. The disturbed state of Roman politics meant that they didn't reach Greece until 87 B.C., but when they did Archelaus soon found himself besieged in Piraeus, while his allies were trapped in Athens.
Both sieges ended in the spring of 86 B.C. – that of Athens after Sulla's men broke into the city and that of Piraeus when Archelaus withdrew to the north to join with a second Pontic army that had advanced into Greece through Thrace and Macedonia. Sulla had responded by moving into Boeotia, where during the summer of 86 B.C. he virtually destroyed the Pontic army at the battle of Chaeronea.
Although Sulla won a crushing victory at Chaeronea this had not greatly improved his position. A new Roman army, under the consul Flaccus, had landed in Greece, officially to attack the Pontic army, but actually to oppose Sulla, whose enemies had seized power in Rome. At the same time Archelaus had received reinforcements – said to be 80,000 strong – under the command of Dorylaus. This gave him an army 90,000 strong, and he still had command of the seas around Greece. From his base at Chalcis Archelaus was able to launch naval raids which reached all the way into the Adriatic.
Sulla took up a position at Melitaea in Phthiotis, where he could watch the route along which Flaccus might approach, while at the same time be able to turn south if Archelaus landed back on the mainland. Sure enough, Sulla soon received news that Archelaus had returned to Boeotia, and was ravaging the countryside. Sulla turned south, ready to fight a second large scale battle.
Archelaus built his camp on the plain of Orchomenus, the biggest in Boeotia, and close to the marshes of Copais, on the shores of the lake of the same name (drained during the nineteenth century). Sulla took up a position facing the Pontic camp. In the centre of the Roman line he posted three lines of infantry with large lanes into which the Pontic chariots could be driven. The men in the second line also built a line of defensive spikes. He then sent out parties to dig ten foot wide trenches on both flanks, to prevent Archelaus from using his cavalry to outflank the outnumbered Roman force.
Archelaus responded by launching cavalry attacks on both flanks. The attack on the Roman left came close to success, and the situation was only saved by Sulla, who rallied his men on foot. Two cohorts taken from the Roman left restored the situation, and allowed Sulla's men to regain their trenches. A second cavalry attack against the line of field fortifications failed.
In the centre Archelaus attacked with his chariots, with the Pontic phalanx in support. As the chariots attacked the Roman front line pulled back behind the spikes planted by the second line. As the chariots became stuck in the spikes, they were attacked by Roman cavalry and light forces who took advantage of the gaps in the infantry line. The chariot horses panicked, and bolted back towards their own lines, disrupting the phalanx. In an attempt to save the situation Archelaus withdrew his cavalry from the attacks on the wings, but this just allowed Sulla to use his cavalry to attack the disorganised Pontic army. Archelaus's troops were driven back into their camp, having lost 15,000 men on the first day of the battle.
On the second day of the battle Sulla continued to dig ditches, this time to besiege the Pontic camp. Archelaus launched another attack on the digging parties, which went so badly that the Romans were able to storm the Pontic camp. The battle turned into a rout, with the surviving Pontic troops attempting to escape into the marshes behind their camp. Archelaus was forced to spend two days hiding in the swamps before he was able to escape to safety, but his army was almost completely destroyed.
When this news reached Mithridates he realised that the war was effectively over, and ordered Archelaus to seek peace terms. Sulla and Archelaus soon agreed terms, and later developed a close friendship. At first Mithridates turned down some of the terms, but his position in Asia Minor began to deteriorate. The army of Flaccus, now under the control of Fimbria, won a victory over Mithridates's son at Miletopolis, while a fleet under Lucius Licinius Lucullus defeated the Pontic fleet at Tenedos (both 85 B.C.). Mithridates then agreed to a personal meeting with Sulla, at which he accepted all of the original terms (peace of Dardanus).