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How did the Thirty Years’ Treaty fail to prevent the Peloponnesian War?

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Thucydides Thucydides manuscript, 3rd century bc (Hamburg, Staats- und Universitätsbibliothek, P. Hamburg 163).

The Thirty Years’ Treaty, established in 445 BCE to end the First Peloponnesian War, failed to prevent the subsequent Peloponnesian War primarily because of escalating tensions between Athens and Sparta’s allies, which eventually drew the major powers into direct conflict.12

According to the historian Thucydides, the underlying cause was Spartan fear of Athenian expansion. The peace was undermined by several key developments:1

Allied Conflicts: Disputes involving Spartan allies, particularly Corinth, created friction. Athens’ intervention in the affairs of Corcyra and Potidaea—both of which had ties to Corinth—pushed the situation toward war.12
Economic Pressure: The Megarian decrees, which imposed an economic embargo on Megara, served as a significant provocation.1
Diplomatic Breakdown: Despite the treaty’s requirement to submit disputes to arbitration, diplomatic efforts failed to resolve these grievances.32

The treaty was effectively shattered when Thebes attacked Plataea, an Athenian ally, in 431 BCE, leading to open hostilities.12