Athens grew; Sparta panicked; Greece bled for decades
From 431 to 404 BC the Delian League under Athens fought Sparta's Peloponnesian League for Greek primacy. Persian money after 413 BC tipped the scales. Lysander's fleet won at Aegospotami in 405 BC, ending Athenian naval supremacy and installing Spartan hegemony.
Thucydides, an Athenian officer exiled in 423 BC, wrote the core narrative from the war's outbreak, combining autopsy with interviews, though his text stops abruptly in 411 BC and he admits the speeches are reconstructions. Xenophon's Hellenica picks up at his last sentence, with a tilt toward Sparta. Diodorus, Plutarch's lives of Pericles, Nicias, Alcibiades, and Lysander, plus archaeology such as Amphipolis's walls, fill gaps. Thucydides's famous line blames Athenian growth and Spartan fear, a fear fed by decades of Delian League expansion after the Persian Wars.
The Archidamian phase opened with King Archidamus II invading Attica. Athens won at Sphacteria in 425 BC, lost to Thebes at Delium a year later, and the exhausted rivals signed the Peace of Nicias in 421 BC. A proxy struggle led by Argos collapsed at Mantinea in 418 BC. After the Sicilian disaster wiped out most of the Athenian fleet, Sparta allied with Persia and, funded by Persian silver, built a navy under Lysander that finally matched Athens at sea. Aegospotami in 405 BC broke the Delian League; Athens fell the next year and got the Thirty Tyrants.
Immediate sparks included Corcyra's appeal after fighting Corinth, the Battle of Sybota, and Athens's harsh demands on Corinthian colony Potidaea. A Thirty Years' Peace had already been strained by the Samian revolt in 440 BC. The costs outlasted the fighting: the Peloponnese sank into poverty, Athens never regained its prewar wealth, and once-ritualised Greek warfare hardened into ideological total war between democrats and oligarchs. Spartan dominance proved brief, since Athens won back its independence in the Corinthian War a decade later.
Source: Peloponnesian War
