The Peloponnesian League was an alliance in the Peloponnesus from the 6th to the 4th centuries BC.
Early history 
By the end of the 7th 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 continued strategies like this to gain other allies in their league. 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 provincial northern Peloponnese joined the league, which eventually included all Peloponnesian states except Argos and Achaea.
League Organization 
The league was organized 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 regardless of that state's size or geopolitical power. No tribute was paid except in times of war, when one third of the military of a state could be requested. Only Sparta could call a congress of the League. All alliances were made with Sparta only, so the member states had to form their own alliances with each other. And although each state had one vote, Sparta was not compelled to abide by any resolutions the League might come to. Thus the Peloponnesian League was not an "alliance" in the strictest sense of the word (nor was it wholly Peloponnesian for the entirety of its existence).
The league provided protection and security to its members. It was a conservative alliance which supported Oligarchies and opposed tyrannies and democracies.
Later History of the League 
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. This might have been caused by Sparta and its allies' unease over Athenian efforts to spread their rule. The two Leagues eventually came into conflict with each other in the Peloponnesian War. Under Spartan leadership, the League defeated Athens and its allies in 404 BC.
Following the disastrous Spartan defeat by Thebes at the Battle of Leuktra in 371 BC, Elis and the Arcadian states seized the opportunity to throw off Spartan hegemony; the Arcadians formed themselves into their own league to preserve their independence. The size of the Peloponnesian League was soon further reduced by the Theban liberation of Messenia from Spartan servitude in 369. The states of the north-eastern Peloponnese, including Corinth, Sicyon and Epidauros, adhered to their Spartan allegiance, but as the war continued in the 360s many joined the Thebans or withdrew into neutrality, though Elis and some of the Arcadian states realigned themselves with Sparta. In 338 BC, the Peloponnesian League was disbanded, when Philip II of Macedon, father of Alexander the Great, formed the League of Corinth after defeating Thebes and Athens, incorporating all the Peloponnesian states except Sparta.