The battle of Mantinea (418 BC) was a Spartan victory over an alliance of Peloponnesian states led by Argos and supported by Athens. The alliance survived into the following year, but the threat that it originally posed to Sparta was gone. The first phase of the Great Peloponnesian War was ended by the Peace of Nicias (421 BC), but this treaty was unpopular with many of Sparta's allies. The Corinthians were particularly opposed to the treaty, and began to agitate against the Spartans. Eventually they convinced the people of Argos to form a new defensive alliance, with the purpose of reducing the power of Sparta. Over the next few years this alliance grew to include Mantinea, the Eleans, and most significantly Athens, but ironically the Corinthians soon lost their enthusiasm for it and remained loyal to Sparta.
Open warfare between Sparta and the new alliance soon began. In the summer of 418 BC the Argives attacked the Epidaurians, Spartan allies in Arcadia (in the Peloponnese). Both sides summoned their allies, and the two armies were soon facing each other outside Argos. A battle seemed inevitable, but instead the two side's leaders met and agreed not to fight. King Agis II led the Spartans back home, where he met a hostile reception. Meanwhile an Athenian contingent joined the Argives and convinced the allies to continue fighting. They captured Orchomenos and then decided to attack Tegea, a key Spartan ally, and in preparation moved to Mantinea.
In Sparta Agis came under attack for his actions at Argos. A committee of ten officers was appointed and he was not allowed to lead an army from the city without their approval. He was saved by the news from Tegea. Messengers arrived announcing that the city would soon change sides if no Spartan army appeared. The Spartans reacted quickly, and Agis was soon in charge of one of the largest armies ever to leave the city. Messages were also sent to Corinth and to the Boeotians, Phocians and Locrians summoning them to Tegea, while the Arcadian allies were collected on the march from Sparta.
One more opportunity for battle would come and go before the actual fighting began. Agis led his Spartans and the Arcadian allies towards Mantinea and began to ravage the local countryside. The Argives responded by forming up for battle in a strong position. Agis formed up his army and advanced to within javelin range, as if he was going to attack, but then changed his mind (possibly because an old soldier in the army warned him that he was trying to make up for his earlier caution with a rash attack). The Spartans pulled back into Tegean territory. Once there they began to divert the flow of some water into Mantinean territory in the hope that this would force the Argives to abandon their strong position.
This plan worked, although the Argive leaders were also responding to pressure not to let the Spartans escape for a second time. The allied army came down off the hill and formed up in order of battle on the plains. They were arranged with the Mantineans on the right, their Arcadian allies next in line, one thousand well trained troops from Argives were next, followed by the rest of the Argive contingent. To their left were the Cleonaeans and Orneans, and finally on the extreme left were the Athenians, with both infantry and cavalry contingents.
The Spartans ran into this allied army while they were returning to their own camp in the plains, and were temporarily caught by surprise. At this point the well-oiled Spartan military machine came to their rescue. Agis was able to order the army to get into formation, and leave the details his well organised troops. The Spartan left was held by 600 Sciritae allies. Next came the soldiers that Brasidas had led in Thrace before his death in 422 BC, and a force of freed helots. The main Spartan contingent was next in line. This was made up of seven regiments, with a width of 448 men. Thucydides was unable to give an accurate figure for the size of either army, but estimated that the Spartan line was normally eight men deep, suggesting that there were around 3,600 Spartans involved. Next came the Spartan's Arcadian allies, then the Maenalians and Tegeans. A small force of Spartans held the extreme right. There were cavalry contingents at both ends of the line.
As the two armies began to advance towards each other, both of their right wings became over-extended. The Mantineans on the Argive side extended beyond the Sciritae, while the Spartans and Tegeans on the Spartan right extended beyond the Athenians. According to Thucydides this happened because the men at the right-hand end of the line tended to drift right in order to prevent their un-shielded right-hand side from being attacked.
Agis responded to this drift by ordering the Sciritae to move to their left to come level with the Mantineans, while two Spartan regiments were ordered to move from the right to fill the gap. The Sciritea moved as orders, but the two Spartan regiments failed to move, leaving a gap in the line. When the battle began the Argives had the edge on this flank, but were defeated everywhere else along the line.
On the Spartan left the Sciritea and Thracians were defeated by the Mantineans. The victorious Mantineans and the 1,000 elite Argives then moved into the gap, and attacked the main Spartan contingent from its left. The victories allies then chased some of the defeated Spartans back to their camp. Meanwhile in the centre Agis was winning. The rest of the Argive contingent, the Cleonaeans, the Ornaeans and the nearest part of the Athenian force were all defeated and began to retreat. Further to the right the Tegeans and Spartans were threatening to outflank the rest of the Athenian contingent. There was now a real change that the Athenians would be surrounded, but they were saved by a combination of their own cavalry and the allied victory on the Spartan left. Agis was forced to respond to this defeat by ordering the rest of his army to come to the aid of his left. This allowed the rest of the Argive force, their allies and the Athenians to escape. The victorious Mantineans and Argives were also forced to flee under pressure from the entire Spartan line.
The Argives and their allies suffered heavy losses during the battle, which was described by Thucydides as the greatest that had taken part between the Greek states for a very long time. The Argives and their local allies lost 700 dead, the Mantineans 200 and the Athenians and Aeginentans 200 (including both of the generals accompanying the Athenian force). Spartan losses were rather lower, at just over 300.
The Spartan victory didn't end the fighting that year. 4,000 reinforcements soon joined the allies, 3,000 from Mantinean allies and 1,000 from Athens. The reinforced allies then attempted to besiege the Spartans in Epidaurus, but without success. This ended the campaign for 418 BC. In the following year the Argives made peace with Sparta, ending their attempt to dominate the Peloponnese. With Argos gone the rest of the anti-Spartan alliance collapsed. Despite having faced each other on the battlefield, Sparta and Athens were still officially at peace, no terms of the Peace of Nicias having been breached, and open war between them didn't resume until 414 BC.