They Just Can't Help It, Can They.-Jermy...
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For more on marking an answer as the "Best Answer", please visit our FAQ.Naval miscalculation, basically. Click http://history.boisestate.edu/westciv/peloponn/16.shtml for further details.
The Athenians lost their fleet at Aegospotami. They didnt guard it properly (even though they were warned to do so) and the Spartans came and burned it.
The loss of the fleet meant that the Spartans could invade, land forces, and take Athens. Which they did in a few weeks. As soon as the news arrived at the Piraeus, the wailing started, because even at the time the athenians realised they had lost the war.