The Peloponnesian War was actually a sequence of three conflicts, fought from 460 to 404 BC,  between Greece's dominant city-states: Athens, master of a sea empire of allied states stretching across the Aegean Sea, and Sparta, which dominated its neighbors on land through the Peloponnesian League.

The two sides were iconic opposites-- Athens was the world's first democracy, where an assembly of male citizens voted on all decisions. Poor in natural resources, Athens was the naval power of the ancient world, and, as long as it held its port at Piraeus, Athens was virtually invulnerable to outside attack. Sparta had the best army in Greece; its male citizens did little but train and fight. But its militaristic elite was hampered by a constant fear of revolt among the helots, the slaves who supported them.

The war began as a regional conflict between the city of Corinth and one of its colonies. Athens and Sparta were drawn into the dispute reluctantly, but as time went on, they found themselves inextricably enmeshed in a conflict which would reduce them both to secondary powers in the Peloponessus.

At the start, Pericles, the great Athenian commander, fought a war of attrition against the Spartans. Instead of risking a land battle by defending the countryside, he brought the entire population into the fortified city and harassed Sparta and its allies with his superior navy. But in 430, a plague broke out within the city walls, killing large numbers of citizens and destroying support for Pericles' tactics. Pericles himself would die of the plague.

The two powers agreed to peace in 424, but neither side held to the treaty. In 415, hostilities broke out again, this time over control of Greek colonies on the island of Sicily. In the following years, the destruction of the Athenian fleet, the revolt of many of Athens' allies, internal unrest, and the intervention of Persia on the side of the Spartans, slowly diminished Athens' power. In 405, Sparta was able to cut off Athens supply lines, and the city was forced to surrender.

The victorious Spartans installed an oligarchy (a government by aristocrats) to rule their defeated neighbor, ushering in a bloody period of witch hunts and political executions. Although that government was overthrown a year later, Athenian democracy was critically diminished. And while Sparta enjoyed a period of dominance in the region, the war left it critically weakened as well. Its hegemony was short-lived.