The battle of Aegospotami (405 BC) was a crushing Athenian defeat that effectively ended the Great Peloponnesian War, leaving the city vulnerable to a siege and naval blockade.
The previous year had ended with a major Athenian victory at the battle of the Arginusae Islands, but in the aftermath of this battle six of the eight Athenian generals had been executed for failing to rescue the survivors from twenty-five ships sunk during the battle, and the remaining two had gone into exile. They were replaced by Conon, Adeimantus and Philocles.
The Spartans also needed a new commander, Callicratidas, the admiral for 406, having been killed during the battle of the Arginusae Islands. At this time it was against Spartan custom to appoint someone to the same post twice, so Lysander, the popular commander of 405, was officially appointed as second in command to Aracus, but in reality it was Lysander who commanded the fleet.
The two sides spent part of the year improving the quality of their fleets, but eventually Lysander decided to move into the Hellespont, partly to try and regain control of a number of cities lost in recent years and partly to try and block the Athenian food supply from the Black Sea. His first success came at Lampsacus, on the Asian shore, which was taken by storm.
When the Athenians discovered that Lysander had moved to the Hellespont, they followed with a fleet of 180 ships. They sailed up the Hellespont, and took up a position at Aegospotami, opposite Lampsacus.
On the next morning the Athenians put out to sea and formed up in line of battle outside Lampsacus. Lysander refused to come out and fight, and after some time the Athenians returned to their base on the beach at Aegospotami. Lysander sent some of his fastest ships to follow the Athenians and discover their routine.
The same pattern was repeated on the next three days. This worried Alcibiades, an Athenian commander in exile for the second time, and he attempted to convince the current Athenian generals to move up the coast to the city of Sestos, where they would have a more secure position.
On the fifth day Lysander made his move. Our two sources disagree on the start of the disaster. In Diodorus Siculus the Athenian commander for the day, Philocles, put to sea with thirty triremes, and ordered the rest of his fleet to follow. Some deserters told Lysander, and he decided to take advantage of the split Athenian fleet. The entire Peloponnesian fleet put to sea, defeated Philocles and then attacked the unprepared Athenian fleet. While Lysander was attempting to capture Athenian ships by dragging them out to sea, a Peloponnesian army was landed on the European shore and captured the Athenian fleet.
In Xenophon Lysander took advantage of Athenian complacency. The Athenians were forced to travel some way to find food, and had got into the habit of dispersing from their ships at the end of each day's sailing. On this day Lysander sent out his fast ships as normal, but this time prepared the entire fleet for battle. When the scouts saw that the Athenians were beginning to disperse they raised a shield as a symbol. Lysander crossed the Hellespont and fell on the disorganised Athenians.
At this point our sources come back together. Conon and nine ships managed to escape from the disaster, but the remaining 170 Athenian ships were all captured. Conon realised that he had lost the war, and sailed into exile on Cyprus.
In the aftermath of this disaster the Athenian position crumbled. Byzantium and Chalcedon were the first of a series of Athenian-held cities to surrender to Lysander, and in each case he allowed the garrisons to return to Athens. News of the defeat was carried to Athens on the state trireme 'Paralus'. With their last fleet gone, the Athenians realised that they were about to be besieged by land and sea, and that they might not expect much mercy if they surrendered. The city was soon surrounded by two Peloponnesian armies and blockaded by Lysander's fleet, and the siege of Athens, the final act of the Great Peloponnesian War, began.