Why did Athens lose the Peloponnesian War?

At the beginning of what has come to be called "the Peloponnesian war," Pericles told the Athenians that -- if Sparta was their only serious foe, if they kept up their fleet, and pursued no ulterior objects — they would "win through." Thucydides believed that the Athenian statesman's prognosis was sound. But, in the end, the Athenians did not "win through." I propose to examine why Pericles' expectations were not met.