The siege of Epidamnus (435 BC) saw the Corcyraeans capture their own former colony, overcoming a garrison partly provided by their own mother city of Corinth (Corinth-Corcyra War, 435-431 BC).
Epidamnus, on the Albanian coast, was a Greek colony founded by Corcyra (modern Corfu). Corcyra was herself a colony of Corinth, and so in keeping with tradition a Corinthian, Phalius, son of Eratocleides, from the then ruling family of the Heraclids, had been selected as the official founder of the city, and the original colonists included a number of Corinthians amongst the Corcyraeans. As with most Ancient Greek cities Epidamnus was the scene of constant strife between the Aristocratic and Democratic factions within the city, and it was also often threatened by the surrounding Illyrians.
In the period just before the siege the Democrats had come to power and had exiled many of the Aristocrats. The exiles had allied themselves with the Illyrians, and began a series of raids on the city. They also attempted to enlist the help of Corcyra, playing on their family connections to the city. The Democrats of Epidamnus also attempted to enlist help from the Corcyra, but with less success, failing to even win an audience.
Their next step was to ask for help from Corinth (after consulting the Oracle of Delphi). The Oracle told them to hand control of their city over to Corinth, an offer than the Corinthians happily accepted. A first group of new colonists from Corinth, Ambracia and Leucas reached Epidamnus safely, marching via Apollonia to avoid the Corcyraean fleet.
The Corcyraeans reacted angrily to the arrival of the new colonists. A fleet of twenty five ships (with fifteen ships following behind) was sent to Epidamnus, where they demanded that the new colonists should be ejected and the exiles allowed back into the city. When the Epidamnians rejected these demands the Corcyraeans joined with the local Illyrians and the aristocrat exiles and began to besiege the city.
When news of the siege reached Corinth a relief force was raised, eventually reaching a strength of 75 ships carrying 2,000 hoplites (and probably a large number of more lightly armed missile troops, recorded as being present at the battle of Sybota two years later). This relief expedition was defeated at the naval battle of Leucimme (435 BC), fought in the seas between the southern part of Corfu and the gulf of Actium.
Even if the Corinthians had been victorious at Leucimme, it would have been too late. The defenders of Epidamnus were already desperate, and on the very same day as the naval battle the city surrendered (given the distance between Leucimme and Epidamnus the two events have to be unrelated). Under the terms of the surrender all Corinthian citizens were held as hostages, while all other foreign troops and settlers were to be sold into slavery.
The two victories on the same day put the Corcyraeans in strong position, which they exploited over the next year, but when it became clear that Corinth intended to continue the fight the previously neutral Corcyraeans decided to attempt to join the Athenian League in order to gain allies in the next stage of the war. This fateful step eventually saw the war between Corinth and Corcyra escalate into the Great Peloponnesian War and drag in most of Greece.