Why did the Athenian expedition against Syracuse end in disaster?

The Athenian Sicilian expedition (415–413 BCE) ended in disaster due to a combination of strategic overreach, command failures, and tactical disadvantages.12

Key factors contributing to the defeat included:

• Leadership Issues: The expedition was initially led by three commanders, but the recall of Alcibiades and the death of Lamachus left Nicias in sole command. Thucydides characterized Nicias as indecisive and incompetent; his illness and superstitious delay following a lunar eclipse prevented a timely withdrawal.132
• Military Disparity: Athens lacked sufficient cavalry to counter the Syracusan forces, leaving their infantry vulnerable during foraging and pursuit.2
• External Intervention: The arrival of the Spartan general Gylippus bolstered the defense of Syracuse, allowing them to block the Athenian siege walls and eventually establish a naval blockade.12
• Tactical Failure: In the final naval battles within the Great Harbour, the cramped conditions negated the Athenian fleet's maneuverability, leading to the total destruction of their ships and the subsequent annihilation of the army during its retreat.12