The Olynthian-Spartan War (382-379 BC) saw the Spartans intervene in northern Greece in an attempt to limit the power of the Chalcidian League.
The Chalcidian League had been formed in around 423 BC, several years after the city of Olynthus in Chalcidice had broken away from the Athenian Empire. The League had survived an attempt to dissolve it at the time of the Peace of Nicias (421 BC). After that little is known of it until around 393, when the new king of Macdeon, Amyntas III, was threatened by an invasion from Illyria. In an attempt to preserve at least some of his kingdom, he asked Olynthus to take over some of his border lands.
By the mid 380s Amyntas felt more secure, and asked Olynthus to return his border lands. Unsurprisingly they refused, and in the fighting that followed advanced further into Macedon, capturing the capital at Pella. In 382 Amyntas sent an envoy to Sparta asking for help. At about the same time ambassadors arrived from Acanthus and Apollonia, near neighbours of Olynthus, complaining that they were being forced into the League, and also asking for assistance.
The Spartans decided to intervene in Thrace. According to Diodorus their motive was a desire to uphold the King's Peace, which included a clause guaranteeing the autonomy of the cities of Greece. Xenophon reports a longer debate, in which the potential threat of an alliance between the Chalcidian League, Athens and Thebes was the main argument for intervention.
The Spartan army moved north in two waves. The Acanthians wanted immediate help, and Eudamidas was sent north with around 2,000 men. His brother Phoebidas followed with the larger part of the Peloponnesian force (Diodorus reverses this order, with Eudamidas replacing Phoebidas after the occupation of Thebes).
Eudamidas reached the Thracian theatre during 382, and was welcomed into Potidaea, clearly an unenthusiastic member of the Chalcidian League. Phoebidas had a more eventful journey. As he passed through Boeotia he was tempted to intervene in Theban politics. Leontiades, one of the Theban polemarchs for the year, is said to have asked for Spartan help against his local rivals, and Phoebidas was happy to occupy the Cadmea (the acropolis of Corinth). In the short term this appeared to be a major Spartan success, but Thebes soon threw off Spartan rule, and it led directly to the decisive Spartan defeat at Leuctra in 371 BC. The reaction in Sparta was mixed - Phoebidas was recalled, put on trial, and fined, but the Spartan garrison remained in Thebes. After his early successes Eudamidas bogged down (probably because most of his army got stuck in Boeotia).
In the autumn of 382 a new army or around 10,000 men, commanded by the harmost Teleutias (a half brother of Agesilaus II), was sent north. He reached Potidaea, and encouraged a more active pursuit of the war. Late in the year the Spartans suffered a near defeat in battle outside Olynthus, but were saved by King Derdas, one of their Thracian allies.
In the spring of 381 Derdas defeated an Olynthian cavalry raid at Apollonia (a northern neighbour of Olynthus), but later in the same year Teleutias was killed in a second battle outside Olynthus while conducting a raid against the city.
Sparta reacted by sending the junior king, Agesipolis I, with thirty advisors. He captured Torone, but then fell ill, and died of a fever in the summer of 380.
Agesipolis was replaced by a new harmost, Polybiades, who arrived late in 380 or early in 379. He was finally able to bring the war to a conclusion. After winning a number of very badly documented battles he laid siege to Olynthius, and probably in the summer of 379 the city surrendered. The Olynthians agreed to become Spartan allies - to have the same friends and enemies as Sparta, and to follow Sparta's lead. The Chalcidian League was also probably dissolved, but this didn't last for long, and the League survived until 348 BC, when it was destroyed by Philip II of Macedon.