411 BC was Athens' worst year before its actual collapse in 404. The catastrophic defeat in Sicily two years earlier had cost the city most of a generation of soldiers and ships. A Spartan fleet under Chalcideus, advised by the brilliant and untrustworthy exiled Athenian Alcibiades, had brought several Ionian cities into revolt. Persia had concluded an alliance with Sparta. At home, an aristocratic faction overthrew the democracy and installed an oligarchy of Four Hundred who began secret negotiations with Sparta. Further rebellions in Rhodes and Euboea, plus the loss of Abydos and Lampsacus on the Hellespont to a Spartan land army, forced the Athenian fleet at Samos to disperse. The Spartan navarch Mindarus then pulled his entire fleet past the Athenians and joined up with the Peloponnesian ships in the Hellespont, basing them at Abydos. The small Athenian force at Sestos fled with losses to Imbros and Lemnos. Athens' grain route from the Black Sea was about to be cut, and the city had no good options.

Thrasybulus took overall command of the Athenian fleet at Samos and brought it to Elaeus on the tip of the Gallipoli peninsula. He spent five days preparing his 76 ships against the 86 Spartan vessels at Abydos. The plan was to sail in a single column up the European shore of the Hellespont while the Spartans would emerge from the Asian shore to engage. The trouble was the geography. The strait narrows past Cynossema and bends, so that ships rounding the point lose sight of the rest of the fleet. When the Athenian left wing under Thrasyllus rounded Cynossema, the Spartans attacked in force. Their plan was to outflank the Athenian right and trap the entire fleet in the narrows, with the center driven aground on the Dog's Tomb. The center grounded almost immediately. Thrasyllus's left was beset by Syracusan allies of Sparta and could not see the rest of the fleet around the bend. Thrasybulus on the right, trying to avoid being encircled, extended his line westward and lost contact with the center. The Athenian fleet was now broken into three pieces and a Spartan victory looked certain.

What saved the Athenians was the worst kind of mistake a winning fleet can make. The Peloponnesian line, smelling victory, broke formation. Individual Spartan ships peeled off to chase Athenian vessels, the way a hound breaks the line of a hunt to chase a single rabbit. Thrasybulus saw it. He turned his ships abruptly into the disorganized Spartan left and broke through. The Athenian right then bore down on the Peloponnesian center, which had its own ships scattered, and routed it. The Syracusans on the Spartan right, watching the rest of the allied fleet fly apart, abandoned their attack on Thrasyllus and fled themselves. By midday it was over. The Athenians lost 15 ships. The Spartans lost 21 plus all their flags and equipment. In tactical terms it was a narrow victory. In every other way it was a deliverance.

After the battle the Athenians refitted their ships at Sestos and sent a small detachment north to Cyzicus, recapturing that town and seizing eight stray triremes along the way. A single trireme was dispatched to Athens with the news. It is hard to overstate what that ship's arrival meant. The democracy in Athens had been overthrown weeks earlier. The oligarchy of Four Hundred had been exposed as negotiating in bad faith with Sparta and had been replaced by a more moderate constitution. The fleet at Samos, where the democratic sailors had refused to recognize the oligarchy, had been the actual seat of legitimate Athenian government for months. The Cynossema victory restored the people's confidence in the war effort and in their navy, gave the moderate constitution political space to consolidate, and led directly to the more decisive Athenian victories at Abydos later that year and the rout of Mindarus at Cyzicus in 410 BC. Athens lost the war eventually. But it lost it later, and harder, because of one tactical victory at the Dog's Tomb.