The map up above shows the long walls from Athens to Piraeus.
The Athenians, instead of fighting a defensive war, which they could have won, have decided to fight a two front war. This is a risky proposition. Hitler tried it, and lost big. We did it in WW II, but had a policy of Europe first, and then Japan. Had it not been for the atomic bomb we would have invaded Japan in November of 1945.
The big problem for the Athenian forces in Sicily was logistics. A modern army is probably even more dependent on good logistics than an ancient one, and the problems of supply involve more items, but the basics, food (enough to feed each man several thousand calories a day), water (1 to 2 gallons per man per day), and gear (armor, ammunition, etc.) were insurmountable. Apparently the ancient custom, which seems bizarre today, was for the soldiers and sailors to buy supplies from the markets of friendly towns.
The recall of Alcibiades, which caused him to go over to the Spartans, did not help either. Alcibiades advice to Sparta and Persia rendered material help to the enemies of Athens.
Thucydides breaks off before the conclusion of the war, but what emerges from the portrait of the Athenian democracy is that direct democracy, as practiced at Athens, was an unstable institution. The people were inclined to punish leaders for not being successful, and were easily swayed by demagogues such as Cleon. They were also superstitious, as shown in their reaction to the mutilation of the Hermae. I think it’s fair to say that Thucydides, whatever his beliefs about democracy, provides illustrations of why democracy is the least desirable form of government.
Next up Plato, Parmenides and probably The Sophist.