Peloponnesian War

The Peloponnesian War was fought between 431 and 404 BCE between two coalitions of Greek city-states: the Delian League, led by democratic and maritime Athens, and the Peloponnesian League, led by oligarchic and land-based Sparta. The primary contemporary account is Thucydides' History of the Peloponnesian War, supplemented for the final years by Xenophon's Hellenica.

Historians traditionally divide the war into three phases:

• The Archidamian War (431–421 BCE), named for the Spartan king Archidamus II, featuring annual Spartan invasions of Attica and Athenian naval raids. A plague struck Athens in 430 BCE, killing Pericles in 429 BCE. The phase ended with the Peace of Nicias in 421 BCE.
• The Sicilian Expedition (415–413 BCE), an Athenian campaign against Syracuse that ended in catastrophic defeat and the loss of much of the Athenian fleet and expeditionary force.
• The Ionian or Decelean War (413–404 BCE), in which Sparta, with Persian financial backing under the satraps Tissaphernes and later Cyrus the Younger, built a fleet capable of challenging Athens at sea. The Spartan admiral Lysander destroyed the Athenian navy at Aegospotami in 405 BCE, leading to Athens' surrender in 404 BCE.

The war is foundational to the study of international relations. Thucydides' attribution of the war's cause to "the growth of the power of Athens, and the alarm which this inspired in Lacedaemon" underpins modern realist theory and the concept of the Thucydides Trap popularized by Graham Allison. The Melian Dialogue (Book V) is routinely cited as an early articulation of power politics: "the strong do what they can and the weak suffer what they must."

The conflict ended Athenian hegemony, installed the short-lived oligarchy of the Thirty Tyrants in Athens, and left the Greek world weakened ahead of Macedonian ascendancy in the following century.