The Corinth-Corcyra War of 435-431 BC began as a dispute between Corinth and her colony Corcyra, but the Athenians were soon dragged into the conflict, and it contributed to the outbreak of the Great Peloponnesian War.
The Corinth-Corcyra War was partly the result of the long-standing hostility between Corinth and Corcyra. Corcyra (modern Corfu) had originally be founded as a colony of Corinth, but for some time the younger city had refused to pay her parent city the usual honours, something that was greatly resented in Corinth.
The relationship between the two cities had not always been so hostile. When Corcyra decided to create a colony of her own at Epidamnus, Corinth had been invited to provide the official 'founder' of the city (Phalius, son of Eratocleides, from the then ruling family of the Heraclids). Corinth also provided some of the original colonists.
The city of Epidamnus was founded on the Illyrian coast, in the territory of the Taulantians (modern Albania). The city had prospered for some time, but in the years before the outbreak of the war had been threatened by both internal conflict and by the Taulantians. Things came to a head when the Democratic faction within the city expelled the Aristocrats. The exiled aristocrats joined with the Taulantians and launched a series of piratical attacks on the city.
Both factions from Epidamnus sought help from their mother city of Corcyra, and the exiled aristocrats were clearly the more successful. The ambassadors from the democrats were refused an official audience, while the exiled aristocrats, who were able to point to the tombs of their ancestors in Corcyra, would soon have the active support of the mother city.
When it became clear to the Democrats that they could not expect any help from Corcyra they decided to consult the Oracle at Delphi to find out if they should ask for help from their founder's city of Corinth. The Oracle replied that they should hand their city over to the Corinthians. Unsurprisingly the Corinthians accepted this offer, and prepared to mount an expedition to the city. A force of colonists from Corinth, Ambracia and Leucas soon reached Epidamnus.
When this news reached the Corcyraeans they responded by sending a fleet to besiege Epidamnus, operating alongside the exiles and the Illyrians. News of the siege reached Corinth, where work began on raising a relief force. This consisted of a military contingent, including thirty ships and 3,000 hoplites from Corinth, and a group of new colonists. A number of Corinth's allies also provided ships, and eventually a force of 75 ships carrying 2,000 hoplites was sent to try and lift the siege of Epidamnus.
While this relief fleet was being put together the Corcyraeans sent a diplomatic mission to Corinth, where they demanded that the new colonists withdraw from Epidamnus, and offered to take the issue to arbitration, with neutral cities from the Peloponnese to serve as the arbitrators. The Corinthians responded by demanding that the siege of Epidamnus be lifted before any negotiations could begin. The Corcyraeans suggested that either both sides should withdraw their troops (and the Corinthians their colonists) or both sides should stay in place while the issue went to arbitration. The Corinthians turned down both of these offers, and the fleet sailed.
As the Corinthians sailed north, the Corcyraeans sent a fleet of eighty ships south. The two fleets met somewhere between the mouth of the Ambracian Gulf (the site of the battle of Actium) and Cape Leucimme (or Leukimme) at the southern end of Corcyra. The resulting battle ended in a victory for Corcyra, after which the surviving Corinthians sailed home. On the same day Epidamnus surrendered.
The first phase of the war thus ended with a clear victory for Corcyra, but the Corinthians were not ready to end the fighting. For most of year after the battle of Leucimme the Corcyraeans were in the ascendency, raiding Corinthian allies from the sea, but all the time the Corinthians were building new ships and preparing to strike back. In the summer of 434 BC the Corinthians occupied a series of fortified positions around Actium, while the Corcyraeans positioned themselves around Leucimme. The two fleets and armies then faced each other across the gulf between Corfu and the mainland for the rest of the summer, only returning to their homes at the start of the winter of 434-433 BC.
Up until this point Corcyra had managed to remain neutral in the affairs of mainland Greece, not joining the Athenian or Spartan led leagues, but as the scale of the Corinthian war effort became obvious they decided to try and join the Athenian League. Corinth also sent representatives to Athens, and the two sides got to put their case to an assembly. Thucydides records speeches from both sides, and although the wording is largely his own, the general arguments are probably the ones used at the time. The Corcyraeans admitted that they hadn't been allies of Athens in the past, but that this was a mistake, and they now needed help to preserve their freedom against a powerful threat. They claimed to be the second most powerful naval force in Hellas, and potentially a powerful ally in any future struggle against Sparta. The terms of the Thirty Year Peace that had ended the First Peloponnesian War expressly allowed any neutral state to join either league. Corcyra was an important staging post on the sea routes to Italy and Sicily, major sources of grain for Athens. Finally the Corcyraeans raised the prospect of Corinth taking possession of their powerful fleet, leaving Athens to face the combined fleets of Corinth, Corcyra and the Peloponnese.
The Corinthians responded by attacking Corcyraean neutrality, describing it as a cover for the wrong-doings of their sailors; accusing them of being a disloyal colony, that they were the aggressors in the war over Epidamnus, and that if Athens did allow Corcyra into their league then war between Corinth and Athens would surely follow. The Corinthians also pointed out that they had recently defended Athens' right to punish her allies when the Spartans had been close to declaring war over the Athenian treatment of Samos.
The Athenians need two assemblies to come to a conclusion, but after the second one they decided to side with the Corcyraeans. This would not be a full alliance, in which each side was bound to come to the aid of the other in any war, but a defensive one, in which Athens was only committed to intervene if Corcyra was attacked. Given than Corinth was clearly preparing for just such an attack, this alliance was just what the Corcyraeans needed. A squadron of ten Athenian ships was sent to Corcyra, with orders to avoid battle unless the Corinthians were attempting to land on Corcyraean territory.
The two fleets were soon facing each other close to the southern tip of Corfu, with the Corinthian fleet anchored in a harbour at Chimerium, on the mainland just to the south of Corfu, while the Corcyraean fleet (and their ten Athenian allies) were a little further north, in the Sybota islands (close to the mainland, opposite the southern tip of Corfu). The Corinthian fleet set sail on the night before the battle, only to find the Corcyraeans already at sea. In the resulting battle of Sybota each side's left wing defeated the other's right, but the Corinthian victory was the more significant. They destroyed 70 ships, the Corcyraeans only 30. After a pause in the fighting the Corinthians were about to return to the fray when twenty fresh Athenian ships were sighted. Fearing that they were the advance guard of a larger fleet the Corinthians withdrew. On the following day they sent envoys to the Athenians, who stated that they would only fight if the Corinthians attempted to attack Corcyra. This allowed the Corinthians to sail home, although only after erecting a victory trophy on the mainland close to Sybota. The Corcyraens also erected a trophy, and perhaps had the better claim to victory, having successfully defended their island against attack by a larger fleet.
After the battle of Sybota the Corinth-Corcyra war lost its intensity, before two years later becoming part of the wider Great Peloponnesian War, in which Corcyra fought on the side of Athens and Corinth on the side of Sparta.