The siege and battle of Corcyra (373-2 BC) saw the defeat of a Spartan attempt to seize control of the Ionian Sea, and triggered a resumption of warfare in the Theban-Spartan or Boeotian War (379-381 BC).
The first phase of the war had seen a series of generally unsuccessful Spartan invasions of Boeotia. Between 379 and 376 the Spartans used the land route into Boeotia, but in 375 they attempted to cross the Corinthian Gulf instead. The Athenians responded by sending a fleet commanded by Timotheus, son of the Athenian naval hero Conon. Timotheus won a naval victory at Alyzeia (June or July 375 BC), but soon afterwards news arrived that peace had broken out. By 375 Athens, Sparta and Thebes were all ready for peace, and they agreed on a similar arrangement to the King's Peace that had ended the Corinthian War. Garrisons and governors were to be withdrawn from occupied communities, and the Greek cities were to be allowed their autonomy.
Almost inevitably the new peace didn't last for long. One of the flash points was the Ionian Sea, where the inevitable clash between oligarchs and democrats was taking place on several of the Ionian Islands, including Corcyra (modern Corfu) and Zacynthus. On his way home Timotheus restored a group of exiled democrats from Zacynthus. Their oligarchic enemies complained to Sparta, who in turn complained at Athens. The Athenians were unwilling to abandon their friends on Zacynthus, and so early in 374 the Spartans sent a fleet of 25 ships of their own to aid the oligarchs.
In the same year a group of exiled oligarchs from Corcyra came to Sparta, and offered to give control of the island to the Spartans if they would help them overcome their democratic rivals. The Spartans sent a fleet of 22 triremes under the command of Alcidas into the Ionian Sea, while claiming that the fleet was heading to Sicily. The oligarchs had overstated the level of support they enjoyed at home, and when Alcidas arrived the port was closed to him.
The Corcyraeans asked for help from Athens. Two forces were sent out. A general called Ctesicles was sent overland through Epirus with 600 peltasts, officially to help the democrats on Zacynthus, while funding was approved to give Timotheus sixty ships for 373. The Spartans responded by raising an equally large fleet of their own, and giving command to Mnasippus, the Spartan Navarch for 373. He was given 60-65 ships, provided by Sparta, Corinth, Leucas, Ambracia, Elis, Zacynthus (showing that the democrats on that island were out of power), Achaea, Epidaurus, Troezen, Hermione and Halieis, and 1,500 soldiers, including a number of mercenaries.
Mnasippus's campaign began well. His arrival at Corcyra caught the defenders by surprise, and he was able to capture four ships in the port. On land part of the Corcyraean army was caught outside the town and wiped out. He was unable to capture the city itself, which was on a peninsula, and had to settle down into a siege. Supplies soon began to run short, and a number of the citizens attempted to leave the city. Mnasippus eventually decided to refuse to let them into his camp, and forced them back towards the city. The defenders were unwilling to let them back in, and so many starved between the walls.
Mnasippus managed to make himself very unpopular within his army. He decided that the siege was almost over, and dismissed some of his mercenaries in order to save money. He also reduced the pay of the remaining troops, and they soon couldn't afford to buy food at the camp market. Instead they had to forage in the local area, weakening the blockade. The defenders were then encouraged by the arrival of Ctesicles and his peltasts. They had reached Epirus, where they had received help from King Alcetas I. He shipped them to Zacynthus, and then to Corcyra, where they were able to get into the besieged city (presumably by sea).
Soon after arriving in the city, Ctesicles led a sortie that killed 200 of the Spartans. This encouraged him to plan a larger scale attack, and picking a moment when the Spartan lines appeared to be especially poorly defended, he led his men and the garrison of the city out to attack the Spartans.
Mnasippus ordered his men to form a defensive line. His Spartans moved into place, but the mercenaries were more reluctant, and some of their officers warned Mnasippus that their men wouldn't fight without food. Eventually the Spartan commander was able to bully enough men into place to form a reasonable line.
The battle began on the Spartan right. Mnasippus's men were able to drive back the attackers, and forced them into a cemetery outside the town walls. The Corcyraeans then took advantage of the buildings within the cemetery, using the tombs as cover and stopping the Spartan advance with missile fire.
The decisive moment of the battle came on the Spartan left. The Corcyraean hoplites on their right attempted to outflank the eight deep Spartan line. Mnasippus attempted to reinforce this vulnerable flank, probably by moving some men from his more lightly engaged centre. The Corcyraeans spotted this movement, and launched a rapid assault on the Spartan lines. The existing Spartan left was hit in the flanks, and the reinforcements in the middle of their manoeuvre, and the Spartan left collapsed. On his flank Mnasippus was faced with an increasing number of attackers, and eventually the Spartan leader was killed. With their general dead, the surviving Spartans fled back towards their camp.
The Corcyraeans failed to take full advantage of their victory, stopping short of the Spartan camp either because they mistook the camp followers for reinforcements, or because they stopped to scoop up the Spartan's slaves and servants. The Spartan second in command, Hypermenes, managed to restore some order in the camp, and he held on until news arrived that the Athenian fleet was finally close by.
The fleet had been delayed by a lack of money. Timotheus had been forced to sail as far north as Thrace in an attempt to gain funds, but without much success. On his return to Athenian waters he was prosecuted by Iphicrates and the politician Callistratus, and put on trial for treason. Iphicrates managed to get Timotheus's old command, and finally took the fleet around the Peloponnese into western waters. Its arrival convinced Hypermenes that the expedition had failed, and he retreated with his fleet and loot. Timotheus was acquitted late in 372 BC, but chose to go into voluntary exile, and entered the employment of the Persian emperor.
The end of the expedition to Corcyra largely ended Spartan interest in the naval campaign. Their best commander, Pollis, was killed in an earthquake at Helice, and in 371 Sparta and Athens once again made peace.