The battle of Eretria (411 BC) was a naval defeat suffered by Athens that was followed by a major revolt on the island of Euboea, cutting the city off from one of its last sources of food (Great Peloponnesian War).
The battle came during a political crisis in Athens. The democracy had been overthrown and replaced by an oligarchy, led by a council of 400. The oligarchs soon began to lose control of the situation, but the passage of the Peloponnesian fleet that would be victorious at Eretria really marked the beginning of the end of their rule.
One of the early actions of the 400 had been to build another wall from Athens to the sea, enclosing a larger area than the Long Walls. Many in the city believed that the real purpose of these walls was to create a space where the Spartans could land, and eventually the hoplites building the wall refused to carry on. The wall was dismantled, and the 400 agreed to pass power onto the 5,000.
The political debate was ended by the appearance of a Peloponnesian fleet of forty-two warships commanded by Agesandridas. This fleet left Megara, on the coast to the south-west of Athens, and sailed east past Salamis. The Athenians assumed that the Spartans were planning to attack the city, and rushed to man their ships.
The Peloponnesians were actually heading for the large island of Euboea, a key part of the Athenian empire. They landed at Oropus, which had been captured by the Boeotians much earlier. The Athenians managed to man a number of ships, although all of their best crews were with the main fleet at Samos, on the opposite side of the Aegean. The Athenian fleet followed the Peloponnesians, and then joined up with another squadron at Eretria, a city on the south coast of the island. The combined Athenian fleet contained thirty-six ships.
In the later stages of the Great Peloponnesian War the Athenians began to pay for the arrogant nature of their Imperial rule. Euboea was on the verge of revolt, and the Eretrians played a trick on the Athenians that would help cost them the battle. After landing at Eretria the Athenian sailors dispersed to find food. Normally they would have been able to shop at the main market, close to the ships, but on this occasion the market was empty, and they were forced to go to the outskirts of the city. A pre-arranged signal was then raised to let the Peloponnesians know when to sail.
Agesandridas was able to take advantage of this ruse. He sailed from Oropus to Eretria, arriving while the Athenians were finding food. The Athenian commanders summoned their crews, only to find they were missing. Eventually the Athenians were able to get to sea, but were forced to fight just outside the harbour.
The Athenians were able to hold their own for a short time, but were eventually forced to retreat and seek safety on shore. Some returned to Eretria, but the city had revolted in their absence, and the crews of those ships were massacred. A total of twenty-two Athenian ships were captured, and their crews captured or killed. Some ships escaped to a nearby Athenian fort.
In the aftermath of this naval defeat Athenian power on Euboea crumbled. A revolt broke out along the entire island, and soon only Oreus remained in their hands. Back in Athens panic broke out. Their main fleet and army was at Samos, and was in rebellion against the oligarchy. The city was divided against itself. There was no fleet left. Euboea, their nearest source of food, had been lost. The Spartan fleet was expected to sail into the Piraeus at any moment.
In fact the war would drag on for another seven years. The oligarchy was quickly overthrown, bringing the fleet at Samos back under control. The Spartans didn't take their chance to attack, and the Athenians would soon win a major victory at Cynossema, allowing them to extend the war.