The Sicilian Expedition is an Athenian expedition to Sicily from 415 BCE to 413 BCE, during the Peloponnesian War.

The expedition is hampered from the outset by uncertainty in its purpose and command structure—political maneuvering in Athens had swollen a lightweight force of twenty ships into a massive armada, and the expedition's primary proponent, Alcibiades, is recalled from command to stand trial before the fleet even reaches Sicily—but still achieves early successes.

Syracuse, the most powerful state on Sicily, responds exceptionally slowly to the Athenian threat, and as a result is almost completely invested before the arrival of a Spartan general, Gylippus, galvanizes its inhabitants into action.

From this point forward, however, as the Athenians cede the initiative to their newly energized opponents, the tide of the conflict shifts.

A massive reinforcing armada from Athens briefly gives the Athenians the upper hand once more, but a disastrous failed assault on a strategic high point and several crippling naval defeats damages the besiegers' fighting capacity and morale, and the Athenians are eventually forced to attempt a desperate overland escape from the city they had hoped to conquer.

This last measure, too, fails, and nearly the entire expedition surrenders or is destroyed in the Sicilian interior.The impact of the defeat is immense.

Two hundred ships and thousands of soldiers, an appreciable portion of the city's total manpower, are lost in a single stroke.

Athens' enemies on the mainland and in Persia are encouraged to take action, and rebellions break out in the Aegean.

The defeat proves to be the crucial turning point in the Peloponnesian War, though Athens struggles on for another decade.

Thucydides observed that contemporary Greeks were shocked not that Athens eventually fell after the defeat, but rather that it fought on for as long as it did, so devastating were the losses suffered.