After the Persian Wars, Athens founded the Delian League, an alliance of over 300 Greek city-states. The League's purpose was to keep fighting Persia, but Athens began to use it as its own personal Empire.
Eventually the enormous growth in Athenian power led to tension with other Greek city-states. Sparta, in particular, contested Athens' rising influence over city-states in the Peloponnesus, the landmass in southern Greece on which Sparta is situated.
In 431 BC, Athens used a brief war between Megara and Corinth to seize control of the Isthmus of Corinth, the narrow strip of land connecting mainland Greece (where Athens was located) to the Peloponnesus. This led to outright conflict with Sparta. At first, Athens' navy gave it an advantage, but the Sparatan military establishment was still unrivaled.
The Spartan general Lysander defeated the Athenian navy at Aegospotami in 405 BC, and Athens surrendered. Greece was then dominated by Sparta for several decades, but the militaristic Spartans proved poor administrators. The Peloponnesian War weakened all the Greek city-states, leaving them vulnerable to ambitious neighbors.
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