The battle of Naxos (September 376 BC) was the first naval victory won by an official Athenian fleet since the end of the Great Peloponnesian War, and saw a fleet besieging Naxos defeat a Spartan fleet sent to lift the siege.
The Theban-Spartan or Boeotian War (379-371 BC) was triggered by the Spartan seizure of Thebes in 382 and the Theban revolt against Spartan control in 379. A series of Spartan attempts to invade Boeotia ended in failure - the campaigns of 379, 378 and 377 at least saw Spartan armies reach Boeotia, but in 376 they didn't even manage that and were stopped at Cithaeron. In the same year the members of the Peloponnesian League decided to change from a land war to a naval one, with the aim of blockading Athens. The Spartans raised a fleet of sixty triremes, and gave command to Pollis, an experienced naval officer. Pollis took his fleet into the Aegean, and attempted to stop a series of grain shipments from reaching Athens.
The Athenians responded by giving Chabrias command of the new fleet of the Second Athenian League. He used part of this fleet to escort the grain ships into port at Piraeus, and then led his fleet south to besiege Naxos in the Cyclades, where the anti-Athenian faction currently held power. Chabrias was given 84 triremes, nearly 20,000 men and a number of siege engines. He landed on Naxos, and began to lay siege to the city, but the attack was disrupted by Pollis, who arrived on the scene with a Peloponnesian fleet of 65 triremes.
Chabrias put to sea, leaving some of his men in the siege lines. The two fleets formed up in the three mile wide gap between Naxos and Paros Islands. The Spartans were deployed with Pollis on their right. On the Athenian side Chabrias commanded on the right and Cedon on the left. Chabrias is said to have gained an advantage by ordering his ships not to display any city insignia. His men were thus free to ram anyone who was displaying an identifying mark, while his enemies had to be sure that they were attacking the right targets.
When the fighting began Pollis gained an advantage on his flank. Cedon was killed, and the Athenian left began to crumble. Chabrias is said to have responded by sending twenty ships from his right, under the command of Phocion, a member of his retinue, to restore the line. Phocion's reinforcements turned the tide on the Athenian left. Chabrias was also victorious on his flank.
Chabrias didn't risk a vigorous pursuit, and instead turned back to rescue the survivors from those Athenian ships that had been sunk, and to recover the bodies of the dead. The battle was clearly hard fought, as the Athenians lost 18 of their 84 triremes. The Spartans lost 24 sunk and 8 captured, a total of 32.
This naval victory restored Athenian control of the Aegean for the first time in some years. Chabrias probably followed up with a raid on Laconia, in which he reached Sellasia, to the north-east of Sparta, and the defeat of the main Spartan naval threat in the Aegean meant that the Athenians were able to send quite a sizable fleet around to the west coast in the following year, where they would win another naval victory at Alyzeia (June or July 375 BC).