The term "Greek Leagues" refers to various alliances formed by city-states in ancient Greece for mutual defense and cooperation. The most notable leagues include the Delian League, led by Athens, and the Peloponnesian League, led by Sparta. These leagues played significant roles in the political and military landscape of ancient Greece. The Delian League was established in 478 BCE to protect against Persian threats, while the Peloponnesian League was formed earlier to counter Athenian power. Tensions between these leagues eventually led to the Peloponnesian War, a protracted conflict that reshaped the ancient Greek world.