The Peloponnesian War between Athens and Sparta and their allies continued for years in a seesaw pattern of victories and defeats. Sparta's victory at the battle of Mantinea in 418 BC, however, seemed to give it the upper hand. Athens decided that it could best recover its momentum by a bold attack across the Mediterranean against two of Sparta's allies, Syracuse on the island of Sicily and Carthage on the North Africa coast.

The Athenian leaders concluded that victory in the West would give Athens renewed strength to return its forces to mainland Greece and crush Sparta.

The Athenian fleet arrived in Sicilian waters in November 415 BC and successfully landed its army in preparation for an assault on Syracuse the following Spring. In April 414 BC the Athenians began constructing a siege wall around Syracuse while their fleet blocked the mouth of the harbor. Their plans quickly unravelled, however. The more capable of the two Athenian leaders was killed in a skirmish. Then the Syracusans engaged the Athenian fleet and dealt it a major defeat, crippling the Athenian supply chain. At that interval a Spartan army arrived to support Syracuse, and the combined forces stymied extension of the siege wall.

Athens responded by sending a second armada in July of the following year, but its troops were mauled in an unsuccessful night attack. The Athenians then compounded their problems disastrously. By late August they had decided to terminate their siege and withdraw, but instead of implementing their plan immediately, they postponed their departure because of superstitious fears caused by an eclipse of the moon on August 27. The Syracusans seized the opportunity to block the mouth of the harbor, bottling up the entire Athenian fleet. In desperation the Athenians abandoned their ships and attempted to flee into the Sicilian interior. They were swiftly overtaken and captured. Those who were not massacred were sent to the Syracusan quarries as slave laborers for the rest of their short lives. The entire Athenian attack force had been annihilated.

Despite its debacle at Syracuse, Athens managed to ward off defeat for another ten years. Athens finally surrendered to Sparta in 404 BC, ending its role as a military power.