The primary cause of the Peloponnesian War, which lasted from 431 to 404 BC, was the intense rivalry between the city-states of Athens and Sparta, along with their respective allies. Athens had established a powerful empire through its Delian League, exerting dominant influence over the Aegean region and accumulating wealth. In contrast, Sparta led the Peloponnesian League, comprised of rival city-states wary of Athenian expansionism. The war stemmed from fear, ideology, and shifting power dynamics, culminating in a protracted conflict that ultimately weakened both city-states and shifted the balance of power in ancient Greece.