The primary reason for the conflict in the Peloponnesian War, which lasted from 431 to 404 BC, was the rivalry and power struggle between Athens and Sparta. Athens, benefiting from its powerful navy and empire, sought to expand its influence across the Greek city-states. In contrast, Sparta led a coalition called the Peloponnesian League, aiming to counter Athens' power and protect its own interests. The war revealed deep-seated tensions, including disagreements over alliances, commerce, and political ideologies. Ultimately, the conflict resulted in devastating losses for both sides and significantly weakened the Greek city-states.