The Peloponnesian War arose out of Sparta's worries about Athenian power in the Aegean.
After the Greeks defeated the Persians in the Persian Wars, Athenian power grew. The Athenians came to dominate the area, creating and heading the Delian League. As Athens became more powerful, both the Spartans and some of Athens's allies became unhappy. Some of the allies rebelled, only to be crushed by the Athenians. Sparta took these events as a sign that there could be trouble for them in the future if Athens came to have too much power. The Spartans worried that they, too, might come under Athenian hegemony. Because of this, the Peloponnesian War started.