The battle of Cyzicus (410 BC) was a major Athenian victory won in the Propontis and that temporarily restored Athenian control of the Hellespont and the sea routes to the Black Sea, as well as restoring confidence and morale in Athens after the disaster at Syracuse (Great Peloponnesian War).
The build-up to the battle began with the main fleets facing each other inside the Hellespont, with the Athenians at Sestus on the European shore and the Peloponnesians at Abydus, on the on Asian shore. Despite their successes in 411 BC (battles of Cynossema and Abydos), the Athenians were still outnumbered in the area, especially after the Peloponnesian admiral Mindarus began to gather all of his available ships in the Hellespont. In order to avoid being trapped and having their fleet wiped out, the Athenians sailed out of the Hellespont into the Aegean, and then around the northern shore of the Chersonese (the Gallipoli peninsula), to Cardia, where they too began to concentrate their fleets.
Mindarus was actually planning to besiege the city of Cyzicus, in the Propontis (the modern Sea of Marmora). He was joined by a Persian army under the satrap Pharnabazus, and the city quickly surrendered. By this point the Athenian commanders Alcibiades, Theramenes and Thrasybulus had all reached the area, and the increasingly confident Athenians believed that they had a chance to fight a decisive battle. Accordingly their fleet sailed back into the Hellespont, passing Abydos at night to hide their numbers, and approached Cyzicus.
Our two sources, Xenophon and Diodorus Siculus, give very different accounts of the resulting battle.
According to Xenophon the Athenian fleet approached Cyzicus under the cover of heavy rain. As the combined Athenian fleet approached the city the rain cleared, and the Athenians sighted the Peloponnesian fleet of 60 ships exercising at some distance from the harbour. The Peloponnesians immediately realised that the Athenians had been reinforced, and pulled back close to the shore, where they prepared to fight a naval battle. Alcibiades sailed past the Peloponnesians with twenty ships and landed. Mindarus followed him, and a land battle followed. Mindarus was killed and his men fled. The Syracusans in the Peloponnesian fleet burnt their ships, but the rest of the fleet was captured. Cyzicus itself fell to the Athenians on the following day.
According to Diodorus the Athenians decided to ambush the Peloponnesians. The Athenian fleet was split into three. Alcibiades was given twenty ships, and was to sail towards Cyzicus, hoping to trick the Peloponnesians in coming out to attack him. He was then to retreat in an attempt to drawn the Peloponnesians further away from safety. The rest of the fleet was divided between Theramenes and Thrasybulus. Their squadrons were to remain out of sight until Alcibiades signalled for them, and they were then to emerge and cut the Peloponnesians off from the shore.
The plan didn’t work entirely as planned. Mindarus came out to chase Alcibiades, and the ambush was triggered, but instead of being trapped at sea, the Peloponnesians fled to the shore near Cleri, where they joined up with a Persian army under the satrap Pharnabazus. A number of Peloponnesian ships were lost during the naval pursuit, and the Athenians then attempted to capture the beached ships, using grappling hooks to pull them off the land. A costly land battle then developed around the ships. Alcibiades was facing Mindarus around the ships, while Thrasybulus also landed his troops and was soon engaged in a battle with Peloponnesians and Persians. Thrasybulus was surrounded and in great danger when Theramenes arrived on the scene. Even after this the battle continued for some time, until the Persian mercenaries began to withdraw. The Peloponnesians were soon forced to follow, and Theramenes was free to come to the aid of Alcibiades. Mindarus was killed during this phase of the battle, and the entire Peloponnesian fleet was captured.
Whichever account of the battle is accurate the result was a crushing Athenian victory. Xenophon quoted a Spartan dispatch home, in which they stated 'the ships are gone. Mindarus has disappeared. The men are hungry. We don't know what to do'.
Athenian control of the Hellespontine region had been restored, the corn supply to Athens from the Black Sea had been secured, and morale in the city began to recover from the setbacks of recent years. Indeed morale in the city recovered to such an extent that a Spartan offer of peace was turned down. This would prove to be a great mistake, for Persian support now meant that the Peloponnesians had the greater resources, and despite a number of further successes the war would soon end in defeat for Athens.