The Peloponnesian League was an alliance of states in the Peloponnese in the 6th and 5th centuries BC.
By the end of the 6th century, Sparta had become the most powerful state in the Peloponnese, and was the political and military hegemon over Argos, the next most powerful state. Sparta acquired two powerful allies, Corinth and Elis, by ridding Corinth of tyranny, and helping Elis secure control of the Olympic Games. Sparta defeated Tegea in a frontier war and offered them a permanent defensive alliance; this was the turning point for Spartan foreign policy.
Many other states in the central and northern Peloponnese joined the league, eventually it included all Peloponnesian states except Argos and Achaea. Spartan superiority was guaranteed when Sparta defeated Argos in battle in 546.
The league was organised with Sparta as the hegemon, and was controlled by the council of allies which was composed of two bodies. The first body was the assembly of Spartiates, and the Congress of Allies in which each allied state had one vote. No tribute was paid except in times of war, when one third of the military of a state could be requested. All alliances were made with Sparta only, so the member states had to form their own alliances with each other.
The league provided protection and security to its members, and most importantly to Sparta. It was a very stable alliance, and supported Oligarchies and opposed tyrannies.
After the Persian Wars the League was expanded into the Hellenic League, including Athens and other states. The Hellenic League was led by Pausanias, but after he was recalled it was led by Cimon of Athens. Sparta withdrew and the Peloponnesian League was refounded with Sparta's original allies, while the Hellenic League turned into the Athenian-led Delian League. The two Leagues eventually came into conflict with each other in the Peloponnesian War.