The battle of Chaeornea (86 B.C.) was the first of two crushing defeats suffered by Pontic armies that ended Mithridates VI's invasion of Greece (First Mithridatic War). During 88 B.C. a large Pontic army under the command of the general Archelaus  had sailed to Athens, and had briefly taken control of much of southern Greece. This success was short lived, and ended when the consul Lucius Sulla landed in Greece at the head of five legions early in 87 B.C. Archelaus and his Greek allies found themselves besieged in Athens  and Piraeus  from the autumn of 87 B.C. until the spring of 86 B.C. Finally, on 1 March 86 B.C. the Romans took the city of Athens. Although the Acropolis held out for a few more weeks, it soon became clear to Archelaus that there was no point defending Piraeus any longer, and he escaped north with his army, to join a second Pontic army that was advancing through Thessaly.
For some time the Romans had had a detachment posted to the north west of Athens under the command of Hortensius, watching the Pontic troops at Chalcis. There was now a real danger that this detachment might be cut off and defeated by Archelaus's newly combined army. Sulla was also running short of supplies in Attica, and so he decided to move north-west into Boeotia, where he could find fresh supplies and also rescue Hortensius.
The Romans were badly outnumbered during this campaign. Most sources give Archelaus 120,000 men. Appian states that the Romans were outnumbered by three to one, giving them around 40,000 men. Sulla himself claimed to only have had 15,000 infantry and 1,500 cavalry, but this was probably pure propaganda. He had five legions present in Greece. Part of this army was still at Athens, besieging the Acropolis, but Hortensius had 6,000 men, and is seems unlikely that Sulla would have risked moving towards Archelaus with less than 10,000 of his own men. We do know that Archelaus was far superior in cavalry, and had a force of at least sixty scythed chariots, then seen as something of a terror weapon, especially when used against infantry.
Hortensius quickly extracted himself from danger, crossing a mountain pass to rejoin Sulla. The combined army then took up a position on the hill of Philoboeotus, at the southern edge of the plain of Elatea, with the Pontic army camped to their north. The two armies must have been separated by the river Cephisus, which runs across the middle of the plain, flowing from west to east. Sulla's position on the hill was strong enough to allow him to refuse battle in such a good area for cavalry, but his biggest problem was that his best line of communication followed the Cephisus as it ran south east out of the plains through a narrow pass towards Chaeronea. The entrance to this pass was guarded by the acropolis of Parapotamii, which stood on the northern (left) bank of the river, at the western tip of Mt. Hedylium.
Both sides appear to have attempted to capture this acropolis, although who moved first is unclear. Sulla's men won the race, blocking the northern entrance to the pass. Archelaus then made a serious error. He decided to try and block Sulla's lines of communication by seizing the city of Chaeronea. Sulla's men on Parapotamii blocked the direct route to Chaeronea, so Archelaus was forced to move around the back of the mountain, approaching Chaeronea from the north.
The course of the battle was dominated by the geography around Chaeronea. The city is located on the southern banks of the Cephisus on a narrow plain, between one and two miles wide. To the south of the town is Mt. Thurium, a hilly area that reaches a maximum height of just over 500m. To the north of the town, on the northern bank of the river, is Mt. Acontium, which runs from west to east alongside the river. A narrow rocky valley separated Mt. Acontium from Mt. Hedylium, a little further to the north.
The people of Chaeronea soon discovered that Archelaus was approaching, and called for help from Sulla. He responded by send a legion under the legate Gabinius which arrived in Chaeronea in time to stop Archelaus from occupying the town. Archelaus took up a position in the valley between Mts. Acontium and Hedylium, and also posted a detachment on the northern slopes of Thurium.
Sulla's main force followed Gabinius down the Cephisus valley, eventually taking up a position to the west of the Pontic camp, on the southern slopes of Hedylium. Sulla waited there for a day, and then moved south onto the plains, leaving a legion and two cohorts under his legate Murena to watch the Pontic camp. Another detachment was sent south, using local guides to reach a position to the south of the Pontic detachment on Mt. Thurium. The two armies were soon drawn up across the river valley. The Roman left under Murena was to the north of the river, facing the Pontic right. The visible Roman right, under Sulla, was to the south of the river, facing the Pontic left.
The battle began when the Roman detachment that had been sent around Mt. Thurium appeared over the top of the hill. The Pontic detachment on the hill panicked and fled down into the river valley, disrupting Archelaus's lines. Archelaus responded with a cavalry charge, which achieved very little, and then by sending sixty of his scythed chariots to attack the Roman lines. When these chariots attacked at speed they were a real threat, but on this occasion they did not have the room to build up any speed, and the Romans let the slow moving chariots pass through gaps in their lines and then killed the crews from behind.
These unsuccessful attacks did at least give Archelaus time to bring his phalanx into the line. A period of intense infantry combat followed in the centre of the line, while the result of the battle was decided on the flanks. Archelaus decided to make a flanking attack with his cavalry against Murena on the Roman left. Sulla responded by sending Hortensius, with five cohorts from the reserve to help Murena, but Archelaus was able to isolate Hortensius, and there was a real danger that the Roman left would crumble.
Sulla reacted by gathering together his cavalry and crossing from the Roman right to the left to help Hortensius. When he saw this Archelaus disengaged his own cavalry, and began to move south to attack to weakened Roman right. Sulla reacted by heading back south, this time with his cavalry, one cohort taken from Hortensius and two cohorts probably taken from the reserve on the right. Sulla reached the southern part of the battle before Archelaus, and launched a full scale attack while the Pontic line was still disorganised. The Pontic left was pushed back towards the River Cephisus. At the same time Hortensius and Murena, on the Roman left, defeated a final Pontic attack and went onto the offensive.
The Pontic retreat quickly turned into a rout. Archelaus's army was split into two halves separated by Mt. Acontium. Archelaus attempted to rally his men in front of their camp between Acontium and Hedylium, closing the gates of the camp, but all this did was increase the death toll. Eventually he was forced to open the camp, but it was too late for most of his army. Only 10,000 men survived to escape to the coast with Archelaus. Roman casualties are unknown. Sulla claimed to have lost only fourteen or fifteen men, two of whom later reappeared.
This crushing victory didn't end the war. Sulla made a dash for the coast with his light troops in an attempt to escape Archelaus escaping across the sea to Chalcis, but without success. On Chalcis Archelaus received 80,000 reinforcements, and was soon able to return to the mainland to seek a second battle. This battle, at Orchomenus, was fought on ideal cavalry ground, but again ended in a major Pontic defeat, which finally ended the war in Greece
