The siege of Plataea (429-427 BC) was a Theban victory that saw them capture Athen's only ally in Boiotia, although only after a two-year long siege.
The city of Plataea was located on the southern edge of Boiotia, the area to the north-east of the Gulf of Corinth. It was the only Boiotian city that was not a member of the Boiotian League (dominated by Thebes), and was instead an ally of Athens. This was not an entirely popular policy inside the city, and two years before the start of the siege these disputes inside the city led to the incident that triggered the Great Peloponnesian War.
In the spring of 431 BC, with the outbreak of war looming, the Thebans decided to try and take control of Plataea. They had the support of one of the political factions inside the city, led by Nauclides, and decided to try and take advantage of this to take the city without a struggle. Two forces were sent from Thebes towards Plataea. The first consisted of 300 men. They were to be let into the city at night and take immediate control. The second, much larger force, would follow some way behind in order to avoid detection, arrive later on the same name and secure Theban control of Plataea.
The first part of the plan worked perfectly. The advance party of 300 men were let into the city and took up a position in the market place. At first the Plataeans were so shocked by the sudden appearance of 300 armed men inside the city that they agreed to the Theban demands, but when it became clear how small the invading force actually was the people of Plataea, including women and slaves, turned against the invaders. In a vicious night fight 120 of the Thebans were killed or escaped from the city, while 180 were taken prisoner. The larger Theban force didn't arrive until the following morning. They then agreed to withdraw from the city in return for the safe return of the prisoners, but as soon as the Theban army was gone the Plataeans executed their prisoners.
This incident ended the last lingering hopes of peace between Athens and Sparta, and triggered the Great Peloponnesian War (431-404 BC). In the first two years of the war the main Sparta effort were two invasions of Attica, but in the third year of the war Sparta and her allies decided to attack Plataea instead. The Plataeans were clearly expecting an attack, for by the time the Spartans arrived the women and children had left the city and had taken refuge at Athens. When the siege began the city was defended by 400 Plataeans and 80 Athenian volunteers, supported by 110 women who cooked for the garrison.
The Spartan army, under the command of King Archidamus, camped outside Plataea while negotiations went on. Both sides tried to use the memory of the Persian War, when Plataea had been the site of the decisive land battle, with the Plataeans calling on the Spartans to honour an earlier oath to guarantee Plataea's independence, while Archidamus called on the Plataeans to either join the Boiotian league or to remain neutral. The Plataeans replied that they would have to consult the Athenians, and that they were worried that the Thebans would use the proposed neutrality in a second attempt to seize the city. Archidamus then came up with a dramatic counter-offer, suggesting that the people of Plataea evacuate their city and let the Spartans occupy it for the duration of the war. They would then be allowed to return once the war was over. Rather surprisingly the Plataean assembly agreed to these terms, but only if Athens approved.
The Plataean envoys returned from Athens with a reassurance that Athens would never abandon them, and a promise that they would provide as much help as possible. This encouraged the Plataeans to turn down the Sparta terms, and the siege finally began.
Archidamus began by constructing a wooden palisade around the city. The Spartans then began to build a mound leading up to the walls, with the intention of mounting an assault. The defenders responded by increasing the height of their own walls opposite the mound. They also dug a tunnel from inside the city to a position under the mound and began to excavate material from inside it, both delaying the construction of the mound and providing themselves with material for their own fortifications. Finally they began work on an inner wall, curving around the area that the mound was heading for so that if the Spartans did manage to cross the outer walls they would be faced with a new line of fortifications.
The Spartans also used more conventional siege engines against the city, but these were ineffective. After this effort had been going on for some time the Spartans decided that they couldn't take the city using their siege engines. Their next step was to wait for a suitable wind and then drop burning bundles into the city from the end of their mound. A large part of the city was destroyed by fire, but the defences remained intact. After this failure the Spartans decided to simply blockade the city. A wall of circumvallation was built around Plataea, most of the besieging army was dismissed, and the blockage began. The Spartan walls were fairly elaborate. Two walls were built (one facing the besieged the city, the other facing outwards to guard against any relief effort), sixteen feet apart. The garrison lived in huts built between the walls. The walls were linked by regularly spaced towers (ten battlements apart according to Thucydides), with gates in the centre of each tower.
By the winter of 428-427 BC supplies were beginning to run out within the city. The defenders decided to try and break out. At first this was to be a mass breakout, but half of the garrison then decided that it was too risky, and only 220 men took part. They used ladders to climb over the inner wall of circumvallation, captured two of the towers, and managed to get across the outer wall before strong reinforcements could arrive. When a mobile force of 300 Peloponnesians did arrive it was too late. The Plataeans then moved up the road leading to Thebes, deceiving the besiegers, who attempted to find them on the road to Athens. Eventually 212 of the escapees managed to reach Athens.
Their efforts were in vain. The Athenians didn't send a relief force to Plataea, as this might have involved them in the formal battle that they were trying to avoid. By the summer of 427 the defenders were so weakened by starvation that they were unable to resist a Spartan attack. At this point the Spartan commander decided to try and negotiate the surrender of the city, on the grounds that conquered places would probably be returned in any peace treaty, while places that surrendered might not be. The Plataeans agreed to surrender and to face the judgement of Sparta, in a fair trial. Instead they were faced with a mock trial, before the Spartans, under pressure from their Theban allies, executed all 200 Plataean and 25 Athenians who had survived the siege. The women who had remained in the city to cook for the garrison were sold into slavery. Plataea itself was demolished, and the building materials used to build a large two-storey hotel and a temple to Hera. The land was then leased out to Thebans. Just as the Spartan commander had expected, Thebes was allowed to keep Plataea in the Peace of Nikias of 421 BC.