The battle of Cynossema (411 BC) was the first major Athenian victory since their disastrous defeat on Sicily in 413 BC, and helped restore morale in the city after a series of setbacks and a period of political upheaval (Great Peloponnesian War).
The battle was caused by a Peloponnesian decision to transfer their fleet from Miletus on the west coast of Asia Minor into the Hellespont, where it could potentially cut Athens off from the grain of the Black Sea. A second reason for the decision was that the Persian satrap of Western Asia, Tissaphernes, hadn't paid the fleet for some time, while Pharnabazus, his neighbouring satrap to the north, was requesting their assistance.
The Peloponnesian fleet, commanded by Mindarus, set out for the Hellespont with 73 ships. It was delayed by bad weather at Icarus, and then moved on to Chios. Thrasylus, the Athenian commander at Samos, responded by moving 55 ships towards the Hellespont, but when he discovered that the Peloponnesians had stopped at Chios, he decided to turn aside to besiege the city of Eresus on Lesbos. There he was joined by a smaller Athenian squadron under Thrasybulus, and some other ships, bringing his fleet up to 67 ships.
The Peloponnesian fleet only stopped at Chios for three days, and then managed to get past the Athenians by sailing between Lesbos and the mainland, reaching Rhoeteum, in the Hellespont, around midnight on the second day after leaving Chios.
There were already two smaller fleets operating in the Hellespont. The Athenians had eighteen ships at Sestos, on the European shore, while the Peloponnesians had sixteen ships at Abydos, on the Asian shore. The Athenians discovered the arrival of the main Peloponnesian fleet just in time to escape from a potential trap, although four ships were lost in a running fight with Mindarus. The Peloponnesian fleets then united at Abydos, giving Mindarus eighty six ships.
The Athenians now had seventy-six ships. They decided to sail into the Hellespont, sticking close to the European shore. The Peloponnesians came out from Abydos, and the two sides prepared for battle.
The battle was fought with the two fleets lined up parallel to the shores of the Hellespont. The Athenians had their backs to the European shore. Thrasylus commanded on the left, furthest into the Hellespont. Thrasybulus commanded on the right, nearest to the open sea. On the other side the Syracusans were on the right, furthest in, and Mindarus with the fastest ships in the fleet was on the left. His plan was to outflank the Athenian right, cutting them off from the open sea, while the rest of his fleet forced the Athenian centre onto the shore.
The Athenians responded to this by extending their right wing, but at the same time the left continued on up the Hellespont, leaving the centre badly stretched out. The Peloponnesians took advantage of this, and forced some ships in the Athenian centre onto the shore. The Athenian soldiers disembarked, and the Peloponnesians followed them, so the battle spread onto land.
The two Athenian wings were holding their own, but were unable to come to the rescue of the centre until the Peloponnesian line began to break up as individual ships broke off to chase retreating Athenians. The two Athenian wings then turned on their direct enemies, defeating them, before combining to crush the Peloponnesian centre. Most of the Peloponnesian ships were able to escape into the Midius River or into Abydos, and only a few ships were captured. Indeed the two sides losses were comparatively similar - the Athenians captured twenty-one ships, but lost fifteen ship themselves - but the real significance of the battle was its impact on Athenian morale, which finally began to recover from the crushing blow on Sicily.