The Peloponnesian War was actually a sequence of three conflicts, fought from 431 to 404 BC between Greece's dominant city-states: Athens, master of an empire of allied states stretching across the Aegean Sea, and Sparta, which dominated its neighborhood 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, it was the naval power of the ancient world, and, as long as it held its port at Piraeus, was virtually invulnerable to outside attack. Sparta had the best army; 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 that would eventually diminish both city-states started 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.

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

The two powers agreed to peace in 424, but neither side held to the treaty. In 415, hostilities started up 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. The city soon surrendered.

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.