The Battle of Coronea in 394 BC was a battle in the Corinthian War, in which the Spartans and their allies under King Agesilaus II defeated a force of Thebans and Argives that was attempting to block their march back into the Peloponnese.
Agesilaus's forces were composed of a regiment and a half of Spartiates, augmented by a force of freed helots, and a sizable force of allied troops from the Peloponnese and Ionia. Facing him on the plain, near the foot of Mount Helicon, was an army made up of Boeotians, Athenians, Argives, Corinthians, Euboeans, and Locrians. In all, the allies probably had 20,000 hoplites. To oppose these, Agesilaus had 15,000 hoplites. The cavalry forces of the two sides were roughly equal, but Agesilaus had substantially more peltasts.
Prior to the battle some of Agesilaus's army were disturbed by an omen witnessed some days before, when the sun had appeared crescent shaped. To reassure his men, Agesilaus first reminded them of the recent Spartan victory at Nemea. He then told them that the Spartan navarch Peisander had been killed in a victory over the Persian fleet. In fact, as Agesilaus knew, Peisander had been killed while suffering a crushing defeat at Cnidus. These reassurances, however, buoyed his army's morale going into the battle.
The defeat at Nemea weighed heavily on the Argives and Corinthians. The Athenians were too familiar with the ups and downs of their previous long and disastrous war against Sparta, and the willingness of the Persians to switch support from one side to the other, to be overly encouraged. Only the Boeotians seemed confident of ultimate victory.
The mercenaries near Agesilaus assumed the battle was over and offered him a garland to commemorate his victory. Just then news came that on the other flank, the Thebans had broken through the Orchomenians and were already at the baggage train, ransacking the loot taken from Asia. Agesilaus immediately wheeled his phalanx around and headed for the Thebans. At that moment, the Thebans noticed that their allies had fled to Mount Helicon. They formed up with the desperate design of breaking through Agesilaus's lines to rejoin the rest of their army.
Agesilaus decided to oppose them by putting his phalanx directly in their path instead of taking them in the rear or flank, a decision that may have been influenced by his longstanding animosity towards Thebes. What followed was evidently one of the worst blood baths in the history of hoplite battles. As Xenophon described it, “So shield pressed upon shield they struggled, killed and were killed in turn.” In the end, a few Thebans broke through to Mount Helicon but, in the words of Xenophon, “many others were killed on their way there.”
According to Diodorus Siculus, more than 600 of the Boeotians and their allies fell, and the Spartans lost 350 men.