The Peloponnesian War was primarily caused by Sparta's fear of the growing power and influence of the Athenian Empire. Following the end of the Persian Wars in 449 BCE, the two powers were unable to agree on their respective spheres of influence in the absence of Persian influence. This disagreement eventually led to friction and outright war. In addition, the ambitions of Athens and its society contributed to increasing instability in Greece.
The ideological and societal differences between Athens and Sparta also played a significant role in the war's outbreak. Athens, the largest maritime power in the Aegean, dominated the Delian League during its Golden Age, which coincided with the lives of influential figures like Plato, Socrates, and Aristotle.
However, Athens gradually turned the League into an Empire and used its superior navy to intimidate its allies, reducing them to mere tributaries. Sparta, as the head of the Peloponnesian League comprised of several large city-states, including Corinth and Thebes, grew increasingly suspicious of Athens' growing power, particularly its control of Greece's seas.