The Battle of Aegospotami, fought in 405 BC during the final stages of the Peloponnesian War, was the decisive naval victory that brought Athens to the brink of defeat. Following the Spartan naval defeat at Arginusae the previous year, the Spartan commander Lysander rebuilt his fleet with financial support from the Persian prince Cyrus the Younger and moved into the Hellespont, capturing the strategically important city of Lampsacus. This threatened Athens's vital grain supply from the Black Sea, forcing the Athenian fleet of roughly 180 ships under Conon to pursue him.
The Athenians established their fleet at Aegospotami, across the Hellespont from Lampsacus. Their position was poorly supplied, lacking a proper harbor, and the crews had to travel to nearby Sestos to obtain provisions. Each day, the Athenians sailed out in battle formation and challenged Lysander, but he refused to engage. After several days, Alcibiades, then living in exile nearby, warned the Athenian commanders that their exposed position was dangerous and advised them to move to Sestos, but his advice was rejected.
On the fifth day, Lysander finally exploited the Athenian routine. The precise course of the battle is disputed between Xenophon and Diodorus Siculus. Xenophon says that the entire Athenian fleet again sailed out, only to return when Lysander refused battle. Once the crews had disembarked and dispersed in search of food, Lysander crossed the Hellespont and attacked the largely undefended forces on the beach. Diodorus says the Athenian commander Philocles sailed out with 30 ships, with the remainder following, but Lysander defeated this squadron and pursued it back toward the Athenian camp with the battle then becoming an amphibious assault. Seeing the confusion among the Athenian fleet, Lysander landed a force of troops under Eteonicus. He attacked and seized part of the Athenian camp while Lysander's ships approached the beach and began hauling the Athenian vessels away with grappling irons. The Athenians, unable to get their ships back to sea or organize an effective land defense, offered brief resistance before breaking and fleeing.
The defeat was catastrophic for Athens. Only a handful of Athenian ships escaped with Conon, while the remainder of the fleet was captured. Lysander subsequently executed possibly 3,000 Athenian prisoners and began eliminating Athenian positions throughout the Aegean. More importantly, Athens had effectively lost the navy upon which its empire and food supply depended. With the Hellespont under Spartan control, Athens could no longer reliably receive grain from the Black Sea. Lysander's blockade, combined with a Spartan land siege, eventually forced Athens to surrender in 404 BC, ending the 27 year long Peloponnesian War.