The battle of Nemea (394 BC) was the first major fighting on the Corinthian front that gave the Corinthian War (395-386 BC) its name, and was an inconclusive Spartan victory.
The Corinthian War (395-386 BC) was the result of simmering tensions between the major Greek powers in the aftermath of the Great Peloponnesian War. Corinth and Thebes felt that they had been denied a just reward for their efforts, and the Spartans didn't help by expanding their power in Thessaly, an area that Thebes felt was within its sphere of influence. The Thebans realised that they would struggle to convince the other Boeotians to fight Sparta without a valid cause, and found one by triggered a conflict between Phocis and Locris, on their western borders. The Phocians invaded Locris, and the Locrians called for help from their Boeotian allies. In turn the Phocians asked for help from Sparta, and war soon broke out.
The first major campaign of the war was a two-pronged Spartan invasion of Boeotia, but this ended after the Spartan left pronge, under Lysander, was defeated at Haliartus (395 BC) and Lysander himself killed. King Pausanias of Sparta was forced into exile after failing to fight to recover Lysander's body, and instead asking for a truce and retreating from Phocis.
The Spartan invasion of Boeotia had convinced the Athenians to enter into an alliance with Boeotia. Corinth and Argos both soon joined the alliance, and the Spartans found themselves facing a major alliance of hostile powers. They summoned King Agesilaus II back from his campaign in Asia Minor, and in the meantime gathered an army, which came under the command of Aristodemus, regent for Pausanias's underage son Agesipolis.
While the Spartans were mobilising, the four allies met at Corinth and decided to launch an invasion of the Peloponnese, on the grounds that the Spartans would be less dangerous if they could be tackled near to home and without their allies. Unfortunately for the allies, they took too long to gather their forces and decide who was to be in command and how deep their battle line should be, and so by the time they were ready to move the Spartans had already mobilised their allies from Tegea and Mantinea and were rapidly approaching.
The initial Spartan target was the town of Sicyon, just inland from the Corinthian Gulf and about twelve miles west of Corinth. By the time the allies were ready to advance, the Spartans had reached Sicyon. The allies may have begun an advance towards the town of Nemea, south-west of Corinth, but if so they were quickly forced to turn north and return to the coast to guard the road to Corinth. The Spartans advanced east through the town of Epieicia, and came under attack from enemy light troops, who were on higher ground to the south. This ended when the Spartans moved onto the coastal plain that bordered the Gulf of Corinth, but it does appear to have given the allies time to get back into place. The allies camped on the eastern side of a ravine (possibly the River Nemea, but that isn't entirely certain). The Spartans pressed east until they were only ten stades away (within two miles), and then camped.
Xenophon gives us figures for the two armies. On the Spartan side he reports 6,000 Lacedaemonian hoplites, 3,000 Eleians, Triphylians, Acroreians and Lasionians (Eleian allies), 1,500 Sicyonians and 3,000 from Epidaurus, Troezen, Hermione and Halieis (in the area east of Argos), a total of 13,500 heavy infantry. There were also 600 cavalry, 300 Cretan arches and 400 slingers. This gives us 14,800 men, to which we probably have to add a sizable force of helots. Other troops were almost certainly present - this list doesn't mention the Tegeans or the Pellenes, but they feature in the battle itself, probably as part of a wider Achaean contingent not recorded in Xenophon's numbers but mentioned in his account of the battle.
The allies had 6,000 Athenians (led by Thrasybulus), 7,000 Argives, 5,000 Boeotians, 3,000 Corinthians and 3,000 Euboeans, a total of 24,000 hoplites. They were supported by 1,550 cavalry and a large number of light troops.
For a few days the two sides faced each other across the ravine. According to Xenophon the Boeotians weren't keen on fighting while they held the left of the line, facing the Spartans, but were much happier when they swapped place with the Athenians. This probably indicates a change in command as well - the allies had agreed to rotate command between the four main powers, and the position of honour at the right of the line probably went with the command for the day.
Xenophon doesn't provide us with details of layout of the two lines on the day of the battle. On the Spartan side we know the Spartans were on their own right, and the Achaeans on their left. On the allied side the Athenians were on the left, facing the Spartans and the Boeotians were on the right, facing the Achaeans. Most of the Allied troops deploying sixteen deep, but the Boeotians adopted a much deeper formation. The details of the battle suggest that the Argives were next to the Athenians, followed by the Corinthians and finally the Thebans.
Once the Athenians had taken the left flank, the allies decided to attack. They advanced across the ravine, but as they went the Boeotians kept drifting to the right. This was often the case in hoplite warfare, as the unshielded right arm was the most vulnerable, but here the Boeotians may have moved deliberately so they could outflank their opponents. The Athenians were forced to follow them in order to prevent a gap developing in the line. Once the Spartans realised a battle was imminent they also began to move to the right, so by the time the fighting began four of the ten Athenian divisions were facing the men of Tegea and only six still faced the Spartans. On their right the Spartans thus outflanked the Athenians, possibly by a very considerable margin.
When the fighting began the Spartans quickly defeated the six Athenian regiments facing them, but elsewhere along the line the allies were victorious and most of Sparta's own allies were soon in full retreat. Xenophon only records one exception, with the Achaeans from Pellene standing firm against the Boeotians from Thespia.
The Spartans are recorded as having encircled the Athenian left and crushing that part of the line. They then moved across the back of the battlefield in an attempt to find fresh victims.
The next allied troops in line were the four Athenian tribes who had defeated the Tegeans. The Spartans passed this position before the Athenians had returned from the pursuit, and so they escaped unscathed. Their first victims were thus the Argives, returning from their own pursuit. The Spartans are said to have let the first few ranks pass them and then attacked them on their unprotected right flank, again inflicting heavy casualties. The same happened to the Corinthians, and then to part of the Theban contingent.
The Allied survivors fled towards Corinth. At first the city gates were closed against them, possible because the defenders expected a Spartan attack, or possibly because the majority of troops left in the city came from the pro-Spartan groups that would soon be exiled. In the event the Spartans withdrew to the point where they first attacked the Athenians and erected a victory trophy. The defeated allies were thus able to recover, and were soon allowed back into the city.
Casualty figures come from Diodorus. According to his figures the Spartans lost 1,100 men, the allies 2,800. However he also gives different figures for the initial strengths of the armies - 15,000 infantry and 500 cavalry for the allies and 23,000 infantry and 500 cavalry for the Spartans (this is very close to the reverse of Xenophon's figures). The only casualty figure in Xenophon is a claim that only eight Spartans were killed.
Although the battle of Nemea was a clear Spartan victory it didn't actually give them much of an advantage. It did stop the allied invasion of Laconia, but with Corinth held against them the Spartans were unable to advance any further. Instead they settled back into their base at Sicyon, and awaited the return of Agesilaus. This would prove to be equally frustrating. He won an inconclusive victory at Coronea (394 BC), but was unable to make any more progress and had to retreat west into Phocis. At the same time the Spartan fleet in Asia Minor was crushed by the Persians (with Athenian help) at Cnidus, ending the Spartan maritime empire).