The Peloponnesian War (431–404 BC) was an ancient Greek war fought between Athens and Sparta and their respective allies for the hegemony of the Greek world. The war remained undecided for a long time until the decisive intervention of the Persian Empire in support of Sparta. Led by Lysander, the Spartan fleet built with Persian subsidies finally defeated Athens and started a period of Spartan hegemony over Greece.

Historians have traditionally divided the war into three phases. The first phase (431–421 BC) was named the Ten Years War, or the Archidamian War, after the Spartan king Archidamus II, who launched several invasions of Attica with the full hoplite army of the Peloponnesian League, the alliance network dominated by Sparta. However, the Long Walls of Athens rendered this strategy ineffective, while the superior navy of the Delian League (Athens' alliance) raided the Peloponnesian coast to trigger rebellions within Sparta. The precarious Peace of Nicias was signed in 421 BC and lasted until 413 BC. Several proxy battles took place during this period, notably the battle of Mantinea in 418 BC, won by Sparta against an ad-hoc alliance of Elis, Mantinea (both former Spartan allies), Argos and Athens. The main event was nevertheless the Sicilian Expedition between 415 and 413 BC, during which Athens lost almost all its navy in the attempted capture of Syracuse, an ally in Sparta.

The Sicilian disaster prompted the third phase of the war (413–404 BC), named the Decelean War, or the Ionian War, when the Persian Empire supported Sparta in order to recover the suzerainty of the Greek cities of Asia Minor, incorporated into the Delian League at the end of the Persian Wars. With Persian money, Sparta built a massive fleet under the leadership of Lysander, who won a streak of decisive victories in the Aegean Sea, notably at Aegospotamos in 405 BC. Athens capitulated the following year and lost all its empire; Lysander imposed puppet oligarchies on the former members of the Delian League, including Athens, where the regime was known as the Thirty Tyrants. The Peloponnesian War was followed ten years later by the Corinthian War (394–386 BC), which, although it ended inconclusively, helped Athens regain its independence from Sparta.

The Peloponnesian War reshaped the ancient Greek world. On the level of international relations, Athens, the strongest city-state in Greece prior to the war's beginning, was reduced to a state of near-complete subjection, while Sparta became established as the leading power of Greece. The economic costs of the war were felt all across Greece; poverty became widespread in the Peloponnese, while Athens was completely devastated, and never regained its pre-war prosperity.[2][3] The war also wrought subtler changes to Greek society; the conflict between democratic Athens and oligarchic Sparta, each of which supported friendly political factions within other states, made war a common occurrence in the Greek world. Ancient Greek warfare, meanwhile, originally a limited and formalized form of conflict, was transformed 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 marked the dramatic end to the fifth century BC and the golden age of Greece.[4]