Having arrived home, I have found the time to read some of Thucydides' history of the Second Pelopennesian War (which is good, since we covered it in class some weeks ago). It is interesting that, though the Athenian invasion of Sicily is widely considered the classic case of "imperial overstretch," all told it had a very good chance of succeeding. Even given the thin support of the local Sicilians against Syracuse, Athens had sufficient resources to threaten the city with capture.
The major reason for the invasion's failure was that certain factions in Athens turned on one of its generals, Alcibiades, and unjustly charged him with crimes against the city. He then defected to Sparta rather than submit to arrest and trial, and advised the Spartans to support Syracuse against Athens. Without this support, Syracuse would almost certainly have fallen; even with it, an aggressive general like Alcibiades may well have won victory where the more cautious Nicias did not.
That civil discord within Athens was the main cause of her defeat is a recurring theme in Thucydides. On its strengths, Athens had every reason to expect victory in the war, as the Spartans acknowledged. Yet because the city could not unify for the greater good, it fell.
Still, we have learned nothing. Note how loudly the mob called for Secretary Rumsfeld to resign over the Abu Ghraib affair.