The siege of Mytilene (428-427 BC) saw the Athenians defeat a revolt on the island of Lesbos, and is most famous for the two debates about the correct punishment for the rebels.
Before the revolt the island of Lesbos was part of the Athenian alliance, but not a member of the more formal Athenian Empire. Instead the different communities on the island had retained their independence, and instead of paying taxes to Athens like the members of the empire, they continued to provide a contingent to the fleet and the army. Despite this greater level of independence the inhabitants of Lesbos had become increasingly concerned at attitude of the Athenians towards the empire. They had wanted to revolt before the outbreak of the Great Peloponnesian War, but had received no support from Sparta and had backed down.
The revolt of 428 BC was led by Mytilene, the largest city on the island. The revolt was carefully planned. Work began on improving the fortifications of the city and on building new warships, while supplies and mercenaries were ordered from Pontus. This time the Mytilenians could rely on support from Sparta and from Boeotia, but they couldn't rely on the support of every community on the island. The people of Methymna on Lesbos, and of the nearby island of Tenedos, remained loyal to Athens, They sent messages to the city to warn the Athenians that the Mytilenians were prepared to revolt and were attempting to unite the island against them.
When this news arrived in Athens the city was suffering from the plague, and so their reaction was delayed. Eventually they decided to send forty triremes to Mytilene, in the hope that they could catch the entire population while they were outside the city celebrating the feat of the Malean Apollo. This plan was betrayed to the Mytilenians, and the Athenian fleet found itself facing a fully prepared city. After a short naval battle outside the harbour the Athenian commanders agreed to an armistice, and a Mytilenian delegation was sent to Athens. At the same time a second mission was sent to Sparta. As expected the embassy to Athens failed, and war broke out between Athens and Lesbos.
The first serious fighting came when the Mytilenians emerged from the city and attacked the Athenians camped nearby. The resulting battle ended with a Mytilenian victory, but they were unwilling to risk camping outside the city, and retreated back inside the walls. The Athenians received some reinforcements from their allies, and built two fortified camps which between them blocked the two harbours of Mytilene, but the land side of the city remained open.
Meanwhile the embassy to Sparta had been successful. The Spartans agreed to come to the aid of Mytilene, and raised a large fleet with the intention of invading Attica while the Athenians were engaged elsewhere. The Athenian managed to raise a fresh fleet of 100 warships, and faced with this unexpected resistance the Peloponnesian fleet withdrew.
The formal siege of Mytilene didn’t begin until the autumn of 428 BC. The Athenian presence outside the city wasn't enough to stop the Mytilenians from campaigning on the island. While the Athenians and Spartans were campaigning near the Isthmus of Corinth the Mytilenians attempted to capture Methymna. The attack failed, as did a counter attack made by Methymna against Antissa, but these events did convince the Athenians that it was time to enforce a proper siege.
At the start of Autumn 428 the Athenians sent 1,000 citizen hoplites, under the command of Paches, son of Epicurus, to conduct the siege. This new army was large enough to complete the blockade of Mytilene. They built a single wall around the city, with forts at key locations. With the harbour already blockaded, the siege lines were now complete.
The Mytilenian's best chance of victory now rested a Spartan fleet of forty ships commanded by Alcidas, but this fleet didn't set sail until the summer of 427 BC. Mytilenian resistance had been bolstered when the Spartan Salaethus had managed to sneak past the Athenian lines to let them know that a fleet was on the way, but this was the only effective help that they received from Sparta. The fleet spent far too long travelling around the Peloponnese and didn't reach Erythrae, on the Ionia coast (the eastern coast of the Aegean) until seven days after the fall of the city. Some members of the expedition wanted to sail to Mytilene anyway in the hope that they would catch the Athenians unprepared to resist an attack, while others wanted to establish a foothold on the Ionian coast, but Alcidas decided to return to the Peloponnese.
The Mytilenians had been forced to surrender by a lack of food. When it became clear to Salaethus that the Spartan relief fleet wasn't going to arrive in time he decided to lead the defenders in an attack on the Athenian lines. The Mytilenians were issued with heavy armour and equipment, but when they realised what Salaethus had in mind they refused to obey him or the city government, and instead demanded that all the food remaining in the city be distributed equally. The city's rulers realised that they were about to lose control of the situation, and entered into negotiations with the Athenians. Paches, the Athenian commander, agreed not to execute, imprison or enslave any of the population until the Athenian people had decided what to do. The Mytilenians were allowed to send envoys to Athens to argue their case, but in return had to agree to submit to whatever punishment the Athenians decided on.
This led to the most famous incident related to the siege. In the first debate on the fate of Mytilene the mood was angry, and the Athenians decided to execute the entire population of the city. A trireme was dispatched to Mytilene with the grim orders. On the following morning the mood had softened, and a second debate was held. Thycicdes records the two main arguments, with Cleon son of Cleaenetus representing the harsh argument and Diodotus, son of Eucrates, the more moderate view. Cleon argued for a reign of terror, in which Athens's allies would be kept in place by fear of the harsh punishment that would follow any revolt. Diodotus argued that this would be counterproductive. Any rebels would know that there was nothing to be gained from surrendering early, and would inevitably fight to the death, making it much more expensive and time consuming to crush even a minor revolt. The moderate view prevailed, and a second trireme was sent to chase the first. The ambassadors from Mytilene promised the crew of this second ship a big reward if they arrived in time. Despite started a full day behind the second ship arrived just behind the first, just after the order to massacre the inhabitants had been read, but before it had been put into effect.
The new terms were much less harsh. Around 1,000 of the leading rebels, who had already been taken to Athens, were executed. The entire island of Lesbos, apart from the lands held by Methymna, was divided into 3,000 holdings. 300 were held to be sacred to the gods, and the rest were distributed by lots to Athenians. These Athenians then charged the locals a rent. Athens also took direct control of the Mytilenian holdings on the Ionian coast.