The battle of Orchomenus (c.352 BC) was the first in a series of defeats suffered by the Phocian leader Phayllus during a failed invasion of Boeotia (Third Sacred War).
Diodorus gives contradictory dates for these events. He places the events in the year of the 107th Olympiad (356 BC), the year in which Aristodemus was Archon at Athens (352-351 BC) and the year in which Gaius Sulpicius and M. Valerius were consuls at Rome (353 BC). A best guess is that he meant the 108th Olympiad, of 352 BC.
In 353 BC the Phocians suffered a very heavy defeat at the hands of Philip II of Macedon at the Battle of the Crocus Field in Thessaly. They are reported to have lost 9,000 men from their army of 20,000, amongst them their leader Onomarchus. They were probably saved from invasion by Philip's decision to settle the affairs of Thessaly before moving south, which gave the Athenians time to garrison Thermopylae. Philip decided not to risk a battle, and returned home.
Onomarchus was replaced by his brother Phayllus, who managed to recruit a new army. He used the loot from Delphi to hire mercenaries. Lycophron and Peitholaus, the defeated tyrants of Pherae, went into exile in Phocis with 2,000 men. According to Diodorus (16.37.3) Sparta sent 1,000 men, Achaea 2,000 and Athens 5,000 infantry and 400 cavalry commanded by Nausicles. These contingents alone would have given Phocis 10,400 men, not counting their own troops and their mercenaries.
Phayllus proved to be less than successful as a military commander. He used his revived army to invade Boeotia, but suffered a series of defeats. The first of them came near the city of Orchomenus, on the north-western shores of Lake Copais. Diodorus provides no details of the battle itself, but does report that Phayllus lost a great deal of men.
This first defeat was followed by further setbacks at the Cephisus River and at Coroneia.