What was the main reason that Athens and Sparta fought the Peloponnesian War?

The main reason Athens and Sparta fought the Peloponnesian War was the growing tension and rivalry between their contrasting political systems and alliances. Athens, with its powerful navy and democratic government, sought to expand its influence through the Delian League, while Sparta led the Peloponnesian League, promoting oligarchic values and military strength. This fundamental clash of ideologies, coupled with competition for control over Greek city-states, ultimately led to the protracted conflict that defined the era.