The battle of Leucimme (435 BC) was a naval victory won by Corcyra over the Corinthians that gave them control of the seas around the western coast of Greece and allowed them to launch raids on Corinth's allies for much of the next year (Corinth-Corcyra War, 435-431 BC). The war was caused by a despite between Corinth and her ancient colony of Corcyra (Corfu) over the right to interfere in the affairs of Epidamnus. That city, on the Illyrian coast, had been founded by Corcyra, but with a Corinthian as her official founder, at a time before the relationship between Corinth and Corcyra broke down.
Like most Greek city states Epidamnus suffered from conflict between Aristocratic and Democratic factions, and was also threatened by the neighbouring Illyrians. Just before the outbreak of the war the Democrats had expelled the Aristocrats from the city. The exiles had united with the Illyrians to attack Epidamnus, carrying out a series of piratical raids. Both factions attempted to gain aid from Corcyra, and the Aristocrats were clearly more successful. After failing to gain aid from their mother city the Epidamnians turned to Corinth, offering to surrender control of the city in return for help. The Corinthians agreed to provide both military aid and new colonists, and the first batch of colonists made their way overland to Epidamnus.
When the Corcyraeans discovered this they laid siege to Epidamnus. In response the Corinthians gathered a relief force, which eventually consisted of 75 ships and 2,000 hoplites. At this date Corcyra was clearly the more important navel power - her fleet alone contained 120 ships, while Corinth could only raise thirty of their own, with the remaining ships coming from their allies.
Once the Allied fleet was ready it set sail for Epidamnus. The Corcyraeans were able to split their fleet, using 80 ships to oppose the Corinthians, while 40 took part in the ongoing siege of Epidamnus. The Corinthian fleet reached Actium, at the mount of the Ambracian Gulf, where they were met by a herald from Corcyra, who attempted to convince them not to attack. When this effort failed, the Corcyraean fleet formed line, and the battle began. Sadly Thucydides, our main source for this campaign, didn't record any details of the battle itself, but only that it ended as a decisive Corcyraean victory. They captured fifteen Corinthian ships, then executed all of their prisoners, apart from the Corinthians, who were kept as hostage. The battle probably took place some way to the north of the Ambracian Gulf, as after the battle the Corcyraeans erected their victory trophy on Cape Leucimme (or Leukimme) at the southern end of Corcyra.
On the same day the defenders of Epidamnus surrendered (although the distance between the two places means that the two events must have been unconnected). These two victories meant that for most of the next year the Corcyraeans had the advantage in the war, and were able to launch raids on Corinth's allies, but about a year after the battle the Corinthians returned to the same area. This time a deadlock developed that lasted until the winter of 434-433 BC, and that encouraged Corcyra to seek an alliance with Athens that eventually saw the war expand to include most of Greece (Great Peloponnesian War).