What alliance did the oligarchies of Greece form for mutual protection?

The oligarchies of ancient Greece formed the Peloponnesian League for mutual protection. This alliance was led by Sparta and included various city-states primarily situated in the Peloponnesian peninsula. The league was established to counterbalance the power of the Delian League, which was dominated by Athens. Ultimately, the Peloponnesian League played a significant role in the Peloponnesian War against Athens.