The siege of Plataea took place in 429–427 BC, during the Peloponnesian War. At the beginning of the conflict, the Thebans attacked the city of Plataea, an Athenian ally on the border between Boeotia and Attica. The initial Theban attempt to capture the city failed, but in 429 BC the Thebans' allies, the Spartans under their king Archidamus II, laid siege to the city. Left unaided by Athens, the Plataeans finally surrendered in 428 BC. Plataea was razed to the ground by the Thebans, and was not restored until after 338 BC by Philip II of Macedon.
First Theban attack
According to Thucydides, an armed force of 300 Thebans commanded by two leading Theban generals were admitted after dark on a stormy moonless night into Plataea by two private citizens who expected the Theban force to immediately capture and kill the democratic leaders and bring Plataea into alliance with Thebes. Instead the Theban commanders harmed no one but attempted to persuade all of the citizens of Plataea to join with Thebes' allies. The democratic leadership accepted the offer but quickly worked out that the invading force could be overpowered because of their small numbers, their lack of knowledge of the streets, the bad weather, and the darkness. The enraged citizenry then attacked them. In one of the rare instances in which both women and slaves took part in what amounted to warfare, the citizenry killed over half the 300 Thebans. Thucydides reports that a number of the remaining Thebans escaped with the help of a Plataean woman who provided them with an axe to break open one of the town's gates. Some of the invaders tried to escape by jumping off the city wall, but most of these were killed in the fall. Others entered a large open building, mistaking it for an exit from the town. The Plataeans locked the building and held them there for a short time before killing them all.
A second larger Theban force was supposed to reinforce the invading force, but the weather and the flooding of the Asopos river delayed them. The Plataeans, to forestall any attempt by the second force to capture Plataeans who lived outside the walls, agreed to let their Theban prisoners live if the Thebans did no further harm. Each side alleged later that the other had taken an oath to the agreement and then broke it, a potentially serious issue in the propaganda war that would impact on future relations between the two sides.
After the invaders had entered the town, a Plataean messenger was sent to Athens to alert them to what was happening. A second messenger later that night updated the Athenians on the outcome. The Athenians sent a messenger back to Plataea instructing them not to kill the prisoners, but he arrived too late. The Plataeans had already killed them all. The Athenians also sent a force to relieve Plataea. Women, children, and men too old or otherwise unfit for military service were evacuated to Attica.
Thucydides states (Book II.1–6) that these events, during which Thebes and its Boeotian allies lost over 10 per cent of their total army, represented the beginning of the Peloponnesian War, which would go on for another 27 years. He says that "the treaty had now been broken by an overt act after the affair at Plataea" and that "Athens and Lacedaemon now resolved to send embassies to the King and to such other of the barbarian powers as either party could look to for assistance."
Spartan intervention and siege
During the summer two years after these events occurred, the Spartan king Archidamus II finally led a Peloponnesian force against Plataea and began to raze their crops. The Plataeans, in response, dispatched a herald reminding the Spartans of the glorious deeds the Plataeans performed during the Greco-Persian War and of the oath the Spartans swore to protect them and keep them independent and reminding the Spartans that in 479 BC Pausanias, the Spartan general, had decreed that Plataea was on holy ground and it should never be attacked. The Spartans responded by demanding Plataean neutrality in return for their protection. After consulting Athens, Plataea rejected the Spartan proposal and began in earnest to prepare their defences. The Spartans then quickly laid siege to the city, and employed several innovative, yet unsuccessful tactics to bypass the Plataean defences. Failing in these undertakings, the Spartans built a line of fortifications around the city, left enough troops to guard the walls, then retired.