Hostility toward Athens brought about the longest war of antiquity, the Peloponnesian War (also called the Great Peloponnesian War to distinguish it from an earlier conflict in the Peloponnesus between 460 and 445 B.C.). In 431 B.C., Athens faced a loose alliance headed by Sparta. The first phase of the war (431-421 B.C.) pitted the most powerful fleet in the Mediterranean (Athens) against one of the strongest armies ever assembled in the ancient world. In this phase, Athens abandoned the countryside of Attica to the invading Spartans, who reinvaded Attica every year attempting to force the surrender of the population within the city walls. After neither strategy gained a decisive advantage, a peace was signed in 421 B.C.

The second phase began in 414 B.C., when Sparta repulsed an Athenian invasion of Sicily. With aid from Persia, Sparta built a large navy that finally destroyed the Athenian navy in 404 B.C. at Aigispotamoi. Thus ended the Athenian Empire and the golden age.

Era of the Polis