The Peloponnesian War's Sicilian Expedition (415-413 BC) is the most instructive strategic catastrophe in ancient history. The Athenians sent their largest fleet and army ever to conquer Syracuse — a city of comparable size, on the opposite end of the Mediterranean, at the end of an impossibly long supply line, without adequate intelligence about Sicilian geography or politics, and without a clear operational plan.

Thucydides' account of the decision-making process is the most analytical surviving account of a democratic decision going catastrophically wrong. The assembly voted for the expedition based on Alcibiades' exaggerated promises and despite Nicias' honest warnings about the logistical difficulties. The very act of Nicias spelling out what the expedition would require — more ships, more men, more money — caused the assembly to vote for exactly the massive force he described.

Nicias, appointed commander against his will and without enthusiasm, exemplifies the problem of competent people unwilling to disobey. His illness, his religious scruples about the lunar eclipse that delayed withdrawal, his failure to adapt plans when conditions changed — all contributed to a defeat that destroyed perhaps 40,000 Athenian and allied soldiers and sailors.

Syracuse was saved largely by the Spartan commander Gylippus, whose energy and tactical skill compensated for the Syracusans' initial vulnerability. The expedition's failure didn't immediately end Athenian power — Athens fought on for another decade — but it destroyed the offensive capability that had made Athens genuinely dangerous to Sparta.