Alkibiades thought the Athenians could take Sicily; he was the main advocate of the expedition and one of the three commanding generals. He proposed that the invaders should work their way around the island subjecting the smaller communities, building up their strength while isolating the most powerful local enemy, Syracuse. With the combined strength of their own forces and those of the whole of Sicily, even mighty Syracuse would not be able to resist them.

But Alkibiades was implicated in a scandal that erupted in Athens just after the expedition had set sail. He was accused of having revealed and mocked the Mysteries of Eleusis at a drinking party, and also of mutilating the herms, sacred boundary stones warding off evil from the city. Both acts were serious offences against the gods and, by extension, against the safety and prosperity of Athens. So Alkibiades was recalled to stand trial, and opted instead to flee to Sparta. In his absence, the remaining two generals (Lamachos and Nikias) soon abandoned his proposed strategy and instead focused their energy directly on Syracuse.

The reasons for the failure of the expedition pretty much all have to do with the nature of Syracuse as a military target.

• The city is very hard to besiege. It has a long wall on the land side (making it difficult to cut off from overland supply trains) and harbours on both sides of the heavily fortified citadel of Ortygia. Due to the rocky terrain surrounding most of the city, the only suitable place for the besieging army to encamp is the marshy ground southwest of the Great Harbour. Armies besieging Syracuse invariably suffer from outbreaks of disease in this unsanitary place. The Athenian army was no exception.

• Syracuse was one of the largest cities of the Greek world both in terms of its physical size (making it harder to surround) and its population. Before the Athenians sent a second fleet of reinforcements, the invaders were outnumbered 2:1 in hoplites and easily matched in naval strength.

• Leading on from that: Syracuse was also one of the few remaining Greek communities to muster a significant fleet of triremes. Most of these states had been gradually absorbed into the Athenian empire in the course of the preceding 60 years, or crippled by war with Athens since the start of the Peloponnesian War. Athens was used to almost complete naval supremacy. At Syracuse, however, it needed half its total fleet just to meet the Syracusans ram for ram. And the longer the Athenian triremes served abroad with no access to drydocks, the more waterlogged and unwieldy they became.

• In addition, Syracuse fielded the largest force of cavalry in Sicily: about 1200 horsemen. Athens had similar numbers at home, but these could not all be sent out without leaving the home territory vulnerable; the Athenians planned to rely on their Sicilian allies to supply them with mounted troops. But since Alkibiades' plan was abandoned, their local allies were fewer than expected, and the expeditionary force could never come anywhere near Syracusan cavalry numbers. As a result, the Athenians were vulnerable in battle, vulnerable in the retreat, and vulnerable when they tried to forage for food from the countryside. Sallying from the city, the Syracusan cavalry swooped down on any exposed target and effectively locked the Athenians up in their camp.

Despite proactive Athenian attempts to build a wall across the heights to blockade the city, to expand their cavalry forces, and to defeat the Syracusan army and navy in repeated battles, everything they did tended to be too little or too late. Syracuse simply held too many advantages to be easily reduced. When Lamachos died in battle, the only remaining Athenian general was Nikias, who was old and ill and had been a strong opponent of the expedition in the first place; all he really wanted was to go home. Meanwhile the Syracusans received help from a Spartan advisor named Gylippos, who further bolstered their defences and streamlined their military command.

Eventually the Athenians sent massive reinforcements along with their most daring and innovative general, Demosthenes, to go and break the stalemate around Syracuse. But his proposed solution (a night assault on the heights overlooking Syracuse) ended in chaos and disaster. After that, the siege became a war of attrition that the Athenians were certain to lose.