Onomarchus (d.353 BC) was a Phocian commander who temporarily halted the advance of Philip II of Macedonia, but who was killed in a battle against the Macedonians in 353. He was the brother of Philomelus, the first leader of the Phocians during the Third Sacred War.
The Third Sacred War (355-346 BC) was fought for control of the oracle at Delphi. In 356 the Amphictyonic Council fined Phocis after finding them guilty of sacrilege, but led by Philomelus, son of Theotimus, the Phocians decided to resist. They occupied Delphi and raised an army. Philomelus used this to defeat an early Locrian attempt to liberate Delphi ( battle of the cliffs of Phaedriades, probably 355 BC), and then achieved success in an invasion of Locris. The Council, led by Thebes, declared a sacred war against Phocis. In 354 the Phocians invaded Locris for a second time, and won two victories, defeating a Locrian and Boeotian army in a cavalry battle, and then a Thessalian army at a hill called Argolas. Philomelus's run of success finally came to an end later in the same year. The Boeotians and Achaeans raised an army 14,500 strong, and defeated Philomelus's 10,000 in an encounter battle at Neon (also known as Tithorea, in the area north of Phocis). Philomelus threw himself off a cliff to avoid capture.
In the aftermath of this disaster command of the remains of the army fell to Onomarchus. Once back in safety he held an assembly of the Phocian people, and convinced them to continue with the war. He also gained official recognition of his command. He was more willing to use the temple goods than his brother had been, and used them to expand his army and to try and bribe some of his enemies. He successfully convinced the Thessalians to become neutral, at least for a short spell.
Like his brother Onomarchus quickly went onto the offensive. He invaded Locris, where he captured the town of Thronium (on the road to Thermopylae), and forced Amphissa to submit. He then moved into Boeotia, where he captured Orchomenus, and moved on to besiege Chaeroneia. This siege ended in failure and he was forced to retreat.
Onomarchus was now dragged into the affairs of Thessaly, which in turn brought him up against Philip II of Macedon. Lycophron, tyrant of Pherae, asked for his help against Philip. Onomarchus sent 7,000 men under his brother Phayllus into Thessaly, but he was defeated by Philip. Onomarchus had to move his main army into Thessaly, where he managed to inflict two rare battlefield defeats on Philip.
Onomarchus then returned south, and invaded Boeotia. He defeated the Boeotians in battle at Hermeum, and captured Coroneia. He was then forced to move north again, to deal with a fresh threat from Philip II, who urgently needed revenge for his earlier defeat.
By this point Onomarchus was able to field 20,000 infantry and 500 cavalry, an impressive force, but unfortunately for him Philip had spent the first part of his reign reforming the Macedonian army, which was now an impressively professional force. At the battle of the Crocus Field (353 BC), Onomarchus suffered a heavy defeat, with the Thessalian cavalry key to Philip's victory.
There are several versions of his death. According to Pausanias he was killed by darts from his own men, who blamed him for their defeat. In other sources he was drowned while attempting to swim out to an Athenian fleet under Chares, which was cruising off the coast. Several thousand of his men died in the same way.
Onomarchus's body was recovered, and Philip had it crucified as posthumous punishment for sacrilege.
Onomarchus was said to have been corrupt, willing to use the temple treasures for his own pleasures. However he had been generally successful in battle until coming up against Philip, and command of the Phocian army passed to his brother Phayllus (at least temporarily). He was able to raise a new force of mercenaries by increasing the pay on offer once again, but rarely had any success in command. After his death the command passed to Onomarchus's son Phalacus, who ended up losing the war.