Excerpts from Books and Wikipedia
"The Sicilian Expedition was an Athenian military expedition to Sicily, which took place during the period from 415 BC to 413 BC (during the Peloponnesian War). The expedition was hampered from the outset by uncertainty in its purpose and command structure—political maneuvering in Athens swelled a lightweight force of twenty ships into a massive armada, and the expedition's primary proponent, Alcibiades, was recalled from command to stand trial before the fleet even reached Sicily—but still achieved early successes. Syracuse, the most powerful state on Sicily, responded exceptionally slowly to the Athenian threat . . . From that point forward, however, as the Athenians ceded the initiative to their newly energized opponents, the tide of the conflict shifted. . . . The Athenians were eventually forced to attempt a desperate overland escape from the city they had hoped to conquer. That last measure, too, failed, and nearly the entire expedition surrendered or was destroyed in the Sicilian interior. . . . The defeat proved to be the turning point in the Peloponnesian War, though Athens struggled on for another decade." [Wikipedia]