The siege of Potidaia (432-430/29 BC) saw the Athenians besiege a city that was part of their empire, and was one of a series of relatively minor military clashes that helped to trigger the Great Peloponnesian War.
Potidaia was an example of a city with a foot in both camps in the clash between Athens and the Peloponnesian League. The city was a colony of Corinth (part of the Peloponnesian Leage), and had maintained friendly relations with the mother city. Potidaia's annual magistrates were still provided by Corinth. At the same time Potidaia was a member of the Delian League, which made it part of the Athenian empire. This would have been an awkward relationship at the best of times, but after the outbreak of near-open hostility between Corinth and Athens during the Corinth-Corcyra War (435-431 BC) the Athenians decided that the Corinthian influence would have to end. There was also a fear that a rebellious Potidaia could trigger a wider rebellion in Thrace, and threaten a key part of the Athenian grain supplies.
Potidaia was located on the Chalcidice peninsula, near to Macedonia. It was at the mainland end of the Pallene isthmus (the western-most of the three narrow isthmuses peninsulas that jut south from Chalcidice. The Athenians made three demands - first that Potidaia send back her Corinthian magistrates and refuse to accept any in future years, second that she send hostages to Athens and third that the city walls facing south towards the Pallene isthmus be dismantled.
The Potidaians responded by sending ambassadors to Athens to argue against these demands, and to Sparta to try and seek allies. They were also aware that King Perdiccas of Macedonia would support them if they did revolt against Athens. Perdiccas was already involved in efforts to convince the Chalcidians to revolt against Athens, and in response the Athenians were on the verge of dispatching a force of 1,000 hoplites against him.
The Potidaian ambassadors to Athens failed to win any concessions, but the ambassadors to Sparta received a promise that if Athens attacked Potidaia then Sparta would invade Attica. When this news reached Potidaia the citizens decided to join with the Chalcidians, and the revolt began.
Soon after this the first Athenian expedition arrived in the area, but its commanders soon realised that they didn’t have enough men to deal with all of their enemies, and decided to focus on Perdiccas and Macedonia first. This gave the Corinthians the time they needed to send 1,600 hoplites and 400 light troops to Potidaea. The Athenians responded with fresh troops, sending 2,000 more hoplites under Callias, son of Calliades. They arrived to find the first army besieging Pydna, and after a time consuming siege were able to come to terms with Perdiccas.
The Athenian army, now reinforced by 600 Macedonian cavalry, marched east along the coast towards Potidaia. By the time they arrived the Macedonians had changed sides once again, and had troops with the Potidaians, Corinthians and Chalcidians (although the original 600 cavalry may have stayed with the Athenians). The allies split their army in two. The Corinthians and Potidaians took up a position on the isthmus just to the north of their city, while the Chalcidians, Macedonians and other allies took up a position at Olynthus, seven miles to the north east. Their plan was to wait for the Athenians to attack the troops outside Potidaia and then attack them in the rear using the forces at Olynthus.
This plan was disrupted by the Athenians, who sent their Macedonian cavalry and some other allies towards Olynthus, preventing the reinforcements from moving. The Athenians then attacked the main allied army. The Corinthian wing of the allied army, under Aristeus, was victorious, but the Athenians won everywhere else along the line. Aristeus was only just able to fight his way back to safety inside Potidaia by advancing in a narrow column along the waterfront.
This battle was a clear Athenian victory. The allies had lost 300 men, the Athenians only 150 (although Callias was amongst them). The Athenians erected a trophy to commemorate the victory, and then began to prepare for a regular siege of Potidaia. At first they only built a wall across the head of the isthmus (to the north of the city), in the belief that they didn’t have enough men to risk splitting their forces by building another wall to the south. When this news reached Athens a third army was dispatched, this time of 1,600 hoplites under the command of Phormio, son of Asopius. This army landed at Pallene, to the south of Potidaia, and advanced along the isthmus. When he reached the city Phormio built a line of siege fortifications to the south, and the city was completely cut off.
Aristeus, the Corinthian commander, believed that the besieged city could no longer expect to hold out. He advised the citizens to evacuate by sea at the first possible opportunity, leaving a garrison of 500 men to defend the city (the same strategy that the Athenians adopted at Plataea). After this advice was ignored, Aristeus escaped from the city and attempted to help the defenders from outside, partly by working with the Chalcidians and partly by called for help from the Peloponnese.
After this dramatic start the siege began to drag on. Thucydides doesn't record any significant events at Potidaia in 431 BC. In the summer of 430 BC the largest Athenian army yet was sent against Potidaia. This force of 4,000 hoplites, 300 cavalry, 100 triremes and fifty ships from Lesbos and Chios was commanded by Hagnon, son of Nicias and Cleopompus, son of Clinias, Pericles's fellow generals. By 430 BC the plague had broken out in Athens, and Hagnon's army took it to Potidaia, where it spread to the troops already engaged in the siege. Hagnon is recorded as having used siege engines against the city, but without success, and the plague killed 1,050 of his 4,000 men. After spending at least a month outside Potidaia Hagnon gave up and took his army back to Athens.
Eventually the siege was ended by starvation. By the winter of 430/429 (or the end of the second year of the war) the situation was so bad that some cases of cannibalism had been record inside the city. The Athenians were clearly also becoming tired with the siege, which had cost them 2,000 talents and forced them to keep a large army in the northern Aegean. This is reflected in the lenient surrender terms that were agreed. The Potidaeans, their wives and children and auxiliary troops were allowed to leave the city in freedom and go anywhere they wished. Each woman was allowed to take two garments with them, each man a single garment, as well as a fixed sum of money for the journey. These lenient terms caused some complaints at Athens, but they did mean that the siege finally came to an end. The Athenians kept possession of Potidaea, eventually resettling the city with their own colonists.