The Peloponnesian War, a 27-year conflict between Athens and Sparta, reshaped ancient Greece. Tensions between the democratic, naval-focused Athens and oligarchic, land-based Sparta erupted into full-scale war, drawing in allies and neutral states alike. The war unfolded in phases, including the Archidamian War, Peace of Nicias, and Decelean War. Key events like the Plague of Athens and the disastrous Sicilian Expedition weakened Athens, while Sparta's eventual naval victory at Aegospotami sealed Athens' fate.