What were the factors leading to the Athenian's defeat in the Peloponnesian War?

The Peloponnesian War was a war between the Greek
city-states of Athens and Sparta.  This war lasted 27 years from 431 B.C to 404 B.C. and
ended with a Spartan victory. In general, Sparta had a powerful army and was able to
control land battles and Athens had a powerful navy and was able to control the sea. 
Sparta was able to eventually win this war for several reasons.  First was a disastrous
Athenian military expedition to Sicily.  Athens wanted to capture the city of Syracuse
and acquire a new source of grain for Athens.  The Athenians lost 200 warships in this
military undertaking, weakening its naval power.  More importantly, Sparta gained an
ally in Persia.  With Persian assistance, Sparta was able to build a more powerful navy,
and was able to defeat and destroy the Athenian navy in the Hellespont.  With this
defeat, Athens lost control of the Aegean Sea. Athens was no longer able to bring food
into the city.  With their food supplies running low, Athens was forced to surrender to
Sparta.  Perhaps the most important factor in the whole war was the aid given to Sparta
by the Persians.