Victor Kioulaphides gave this succinct explanation of the downfall of Athens on the Yahoo Epicurean discussion group site.  It has eerie echoes of our current problems:

The demise of ancient Athens was the hapless Sicilian campaign. False data:
 
A. Athenians thought it took about one day to sail around Sicily; it took triremes no less than *three*, WITH weather (always fickle) on their side.
 
B. Athenians thought it took three days to march across Sicily; it took heavy infantry no less than a whole *week*.
 
C. Athenians thought Sicilians would rise up and join them against Syracuse, the Big, Bad Guy of the island. Think twice: Syracuse was IN Sicily, its natives WERE Sicilians, they were in no position to occupy and rule ALL Sicily (and other Sicilians *knew* it) etc.; Athenians were the foreign invaders.
 
D. Athenians thought land-locked Sparta would sit silent, impotent. All it took was some handy, naval assistance, plus some of Lysander’s craftiness –the "Fox of Sparta"– to tilt the scales. The rest is history.