The siege of Scione (423-421 B.C.) came after the city rebelled against Athens, with Spartan support, but continued on after those cities agreed a short-lived peace treaty, and at the end the defenders of the city were either executed or sold into slavery.
Scione was located in Pallene, the western-most of the three narrow peninsulas that jut south from Chalcidice, in the north of modern Greece (south of Thessalonica). The city of Potidaia, at the head of the Pallene peninsula was held by Athens, and for the first few years of the Great Peloponnesian War Scione was an Athenian ally. This changed in 423 BC, when encouraged by the success of the Spartan general Brasidas in northern Greece the people of Scione decided to revolt.
When he learnt of the revolt Brasidas crossed over to Scione, where he made a speech and left a garrison. This was soon strengthened, and Brasidas hoped to use the city as a base for an attack on Potidaia. The revolt of Scione came two days after a one year long armistice was agreed between Athens and Sparta. At first Brasidas claimed that the revolt had taken place before the armistice, which would thus have included Scione, but the Athenians refused to accept this and prepared to besiege the city. Brasidas evacuated the women and children from the city and prepared for a siege. The garrison was reinforced with 500 Peloponnesian hoplites and 300 Chalcidian peltasts, and was commanded by Polydamidas. Brasidas then departed to campaign in Macedonia.
While Brasidas was away the Athenians made their move. A force of fifty ships, 1,000 hoplites, 600 archers, 1,000 Thracian mercenaries and a small number of peltasts sailed down the peninsula, captured the town of Mende, and advanced on Scione.
As the Athenians approached the defenders of Scione occupied a strong position on a hill outside the city. The Athenians were aware that they would have to capture this hill before they could begin the siege, and took it by frontal assault. They then erected a trophy to commemorate the victory and began to build walls of circumvallation around Scione. The walls were completed by the end of the summer of 423 BC. The Athenians left a garrison to man their walls and the rest of the army returned home.
In the summer of 422 BC, after the year-long truce expired, the Athenian leader Cleon was appointed to lead an army in Thrace. He visited Scione on his way, but instead of helping end the siege he took some of the besieging troops to reinforce his own army.
The siege lasted into the summer of 421 BC. By the time it ended the war between Athens and Sparta had been temporarily ended by the Peace of Nicias (421 BC). Under the terms of the peace treaty any men sent by Brasidas, Spartans or allies of Sparta besieged in Scione were to be released by the Athenians, but the inhabitants of the city were excluded from the treaty. When the city finally fell the Athenians treated the defenders with the harshness that was becoming standard during the war. All men of military age were executed, while the women and children were sold into slavery. The city and its lands were given to the Plataeans, allies of Athens who had lost their own city after the siege of Plataea (429-427 BC).