Wouldn't want this sub to fail you!

The trouble is that we don't really know. Our main source for the expedition - the Athenian historian Thucydides - essentially wrote a lengthy account of how the whole thing was a terrible idea and the defeat of the expedition was the greatest disaster ever suffered by any Greek city ever. Understandably, this picture may not be entirely fair on all involved.

In Thucydides' version, the expedition had various pretexts: the Athenians were recovering from the plague and ready to resume the expansion of their empire, and the subjugation of Sicily could arguably prevent future support to Sparta from the Dorian Greeks who lived there. The Athenians seem to have been especially worried about Syracuse supplying grain to the Peloponnese to allow it to weather Athenian raiding in wartime. Whether such shipments were ever actually sent before the Sicilian Expedition is not known, but we do learn from Thucydides that the Athenians had been meddling in Sicilian affairs since at least 427, and clearly had designs on that quarter for one reason or another. They had allies in Egesta (Segesta) on Sicily, whom they could plausibly claim to be "assisting" by invading the island.

The real reason, again according to Thucydides, was plain greed. Athenians were pathologically ambitious and always wanted to expand further and further. Sicily was a rich land, and Syracuse a natural rival to Athens that should be taken care of before it grew too strong. Meanwhile, some were allegedly even spinning tales of taking on the Carthaginians after the conquest of Sicily, and expanding across the entire Western Mediterranean. Such dreams appealed to the Athenian Assembly. The militia hoped to profit from the loot, the poor who rowed in the fleet hoped to secure a regular wage, and the Athenians as a whole hoped to further glorify their city with the increased tribute that conquest of Sicily would bring.

Thucydides has the sly manipulator Alkibiades persuade the assembly to embark on the expedition against their own better judgment. In effect he is blaming both the democratic system and the people at large for allowing the expedition to happen. Needless to say, this is all hindsight, and the Athenians may well have believed that expansion into Sicily was both logical and doable. They certainly had a good track record when it came to subduing islanders, from Thasos to Aigina to Samos to Lesbos. They certainly misjudged the wealth and power of Syracuse, but then, the strategy suggested by Alkibiades (of first taking over the whole of the rest of the island before turning on an isolated Syracuse) actually seemed to take the difficulties into account. The expedition may have been less a matter of doomed hubristic imperialist greed, and more a matter of ruthless calculating imperialist greed.