The siege of Mantinea (385 BC) saw the Spartans take advantage of their dominant position in Greece after the end of the Corinthian War to attack one of their long standing local rivals and a half-hearted ally in the recent war.
Mantinea was located about fifty miles from Sparta. The city had been formed by the merger of five villages, possibly as recently as the previous century, and had joined the Peloponnesian League. During the Great Peloponnesian War she had sided with Athens, and after the end of that war had been forced back into Sparta's league. During the Corinthian War Mantinea had remained on the Spartan side, but hadn't displayed any great enthusiasm. The Spartans were never terribly keen on having Mantinea that close to their borders, especially as the city was a democracy. The Spartans also objected to the construction of brick city walls around Mantinea, but without success.
After the end of the Corinthian War the Spartans demanded that the Mantineans should dismantle their walls, claiming that they had failed to fulfil their duties as members of the Peloponnesian League. The Mantineans refused to comply, and began to prepare for a siege. An attempt to gain Athenian aid failed, after the Athenians claimed that intervening would breach the terms of the recent King's Peace, although the Spartan attack was itself probably a breach of the same treaty. The Spartan attack probably took place fairly late in the year, as the Mantineans were well stocked with food after a good harvest.
The Spartans gathered a Peloponnesian League army, which was said to have included a Theban contingent in which the future leaders Pelopidas and Epaminondas took part. The invasion was led by the young King Agesipolis I, after his more experienced co-monarch Agesilaus II refused to take part on the grounds that the Mantineans had helped his father against Messenia in 464.
Agesipolis invaded Mantinean territory, and after the obligatory destruction of the surrounding countryside attacked the city itself. An attempt to storm the defences failed, and the Spartans then settled into a regular siege. Agesipolis had his men dig a trench all around the city, using half of his men to do the digging and the other half to guard the diggers. The ditch was followed by a rampart, until Mantinea was totally blockaded.
The good harvest meant that the defenders were undaunted, and continued to defy the Spartans. They also had some supporters in the Peloponnesian army, possibly worried that they would soon share the same fate. These supporters smuggled some supplies into the besieged city, until Agesipolis brought in guard dogs to block off this last supply route.
In order to speed up the siege, Agesipolis ordered a stream that flowed through the city to be dammed downstream of Mantinea. The resulting lake flooded the town and began to undermine the brick defensive walls. Attempts to prop them up failed, and the Mantineans offered to obey the Spartan demands and demolish the walls. With the walls already on the verge of collapse Agesipolis now upped his demands, and insisted that the city of Mantinea should be dissolved back into its original five villages. One of the terms of the King's Peace had been a guarantee of the autonomy for each Greek community, and the Spartans are said to have used this to justify the separation.
The Mantineans had no choice other than to accept these peace terms. Agesipolis's exiled father, the former king Pausanias, arrived from his exile at Tegea just in time to save the lives of the leading democrats, with whom he had a family connection. Sixty leading Democrats went into exile, the city walls were destroyed, and the citizens of the city returned to their original villages. Over the next few years they acted as loyal allies of Sparta, and provided hoplites for the Peloponnesian League up to the battle of Leuctra (371 BC). This battle saw the beginning of the end of Spartan power, and in one of the Theban invasions of the Peloponnese that followed the city of Mantinea was restored.
Some accounts of this campaign also include a major field battle, in which Pelopidas and Epaminondas fought on one flank of the Peloponnesian army, where they suffered a heavy defeat and were only saved from death by King Agesipolis.
The attack on Mantinea was one of a series of high handed Spartan actions that alienated the rest of Greece. The most serious of these was the seizure of power in Thebes in 382, carried out by a Spartan army on its way north to fight at Olynthus. This soon triggered an anti-Spartan uprising in Thebes, starting the Theban-Spartan War (379-371 BC). That war ended with the crushing Spartan defeat at Leuctra (371 BC), the battle that marked the start of a dramatic collapse in Spartan power.