The Peloponnesian War
The Delian League was not the only alliance in Greece. After the Persian Wars, many cities in south Greece, including Sparta, banded together as well. This alliance was called the Peloponnesian League after the peninsula on which on which the cities were located.
The growth of Athenian power worried many cities in the Peloponnesian League. Finally, to stop Athenians growth, Sparta declared war.