The Peloponnesian War (431–404 BC)

This devastating conflict between Athens and Sparta reshaped ancient Greece. Fueled by political rivalry, trade disputes, and Athenian imperialism, the 27-year war saw Sparta's land-based dominance clash with Athens' naval supremacy. Despite early Athenian victories, a failed Sicilian Expedition (415–413 BC) and Persian-backed Spartan forces turned the tide. Athens' surrender in 404 BC marked the end of its Golden Age and weakened all Greek city-states. The war's detailed chronicle by Thucydides became a foundational work in military strategy and political science, highlighting the destructive nature of prolonged conflict.