Page "History of Greece" Paragraph 36

Those who signed the Peace of Nicias in 421 BC swore to uphold it for fifty years.

The second stage of the Peloponnesian War began in 415 BC when Athens embarked on the Sicilian Expedition to support an ally ( Segesta ) attacked by Syracuse and to conquer Sicily.

Initially, Sparta was reluctant, but Alcibiades, the Athenian general who had argued for the Sicilian Expedition, defected to the Spartan cause upon being accused of grossly impious acts and convinced them that they could not allow Athens to subjugate Syracuse.

The campaign ended in disaster for the Athenians.