The major cause of the Peloponnesian War was the power struggle between Athens and Sparta, leading to rivalries within the Greek city-states. After the Persian Wars, Athens emerged as a dominant naval power, forming the Delian League. Sparta, feeling threatened by Athenian imperialism and the increasing tension among states, began to rally allies, culminating in a series of conflicts. The immediate triggers included disputes over trade and influence, particularly in Sicily, and incidents like the Corcyrean Affair. The war lasted from 431 to 404 BCE and resulted in significant devastation and the eventual downfall of Athenian power.