The Peloponnesian War is a Greek military conflict, fought by Athens and its empire against the Peloponnesian League, led by Sparta.

Historians have traditionally divided the war into three phases.

In the first, the Archidamian War, Sparta launches repeated invasions of Attica, while Athens takes advantage of its naval supremacy to raid the coast of the Peloponnese attempting to suppress signs of unrest in its empire.

This period of the war is concluded in 421 BCE, with the signing of the Peace of Nicias.

That treaty, however, is soon undermined by renewed fighting in the Peloponnesus.

In 415 BCE, Athens dispatches a massive expeditionary force to attack Syracuse in Sicily; the attack fails disastrously, with the destruction of the entire force, in 413 BCE.

This ushers in the final phase of the war, generally referred to either as the Decelean War, or the Ionian War.

In this phase, Sparta, now receiving support from Persia, supports rebellions in Athens' subject states in the Aegean Sea and Ionia, undermining Athens' empire, and, eventually, depriving the city of naval supremacy.

The destruction of Athens' fleet at Aegospotami effectively ends the war, and Athens surrenders in the following year.The Peloponnesian War reshapes the Ancient Greek world.

On the level of international relations, Athens, the strongest city-state in Greece prior to the war's beginning, is reduced to a state of near-complete subjection, while Sparta is established as the leading power of Greece.

The economic costs of the war are felt all across Greece; poverty becomes widespread in the Peloponnese, while Athens finds itself completely devastated, and never regains its pre-war prosperity.

The war also wreaks subtler changes to Greek society; the conflict between democratic Athens and oligarchic Sparta, each of which support friendly political factions within other states, makes civil war a common occurrence in the Greek world.Greek warfare, meanwhile, originally a limited and formalized form of conflict, transforms into an all-out struggle between city-states, complete with atrocities on a large scale.

Shattering religious and cultural taboos, devastating vast swathes of countryside, and destroying whole cities, the Peloponnesian War marks the dramatic end to the fifth-century-BCE golden age of Greece.