The Peloponnesian League was an alliance in the Peloponnesus from the 6th to the 4th centuries BC, dominated by Sparta. It is known mainly for being one of the two rivals in the Peloponnesian War (431–404 BC), against the Delian League, which was dominated by Athens.
By the end of the 7th century BC Sparta had become the most powerful city-state in the Peloponnese and was the political and military hegemon over nearly all of the Peloponnese, with the only challenge to the city being Argos, the next most powerful city-state. Sparta acquired two powerful allies, Corinth and Elis (also city-states), by ridding Corinth of tyranny, and helping Elis secure control of the Olympic Games. Sparta continued to use aggressively a combination of foreign policy and military intervention to gain other allies. Sparta suffered an embarrassing loss to Tegea in a frontier war and eventually 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.
The League was organized with Sparta as the hegemon, and was controlled by the council of allies which was composed of two bodies: the assembly of Spartiates and the Congress of Allies. Each allied state had one vote in the Congress, regardless of that state's size or geopolitical power. No tribute was paid except in times of war (mainly against the Delian League), 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 if they so wished, member states had to form separate alliances with each other. Wars among the members were possible as well, with Thucydides mentioning a battle between Mantineans and Tegeans (and their respective allies) during the Peloponnesian War. And although each state had one vote, League resolutions were not binding on Sparta. In addition, the "one state, one vote" principle allowed Sparta to often ensure a formal majority through the smaller towns it could dominate directly.
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 common name used in contemporary documents was "The Lacedemonians and their allies", emphasizing the leadership of Sparta. However, other poleis could hold influence comparable to Sparta herself, especially Corinth, due to its wealth and navy.
The League provided protection and security to its members. It was a conservative alliance which supported oligarchies and opposed tyrannies and democracies.
During the Persian Wars, the League was expanded into the Hellenic League and included Athens and other states. The Hellenic League was led by Pausanias and, after he was recalled, by Cimon of Athens. After the Persian Wars, Sparta withdrew from the Hellenic League, reforming the Peloponnesian League with its original allies. This might have been caused by Sparta and its allies' unease over Athenian efforts to increase the power of their own Delian League.
Tensions between the two Leagues were key in the outbreak of the First Peloponnesian War in 460 BC. The conflict between two Peloponnesian League members, Corinth and Megara, specifically the latter's defection to the Athenians due to perceived neglect by the Spartans, was a key factor in the outbreak of hostilities between the two Leagues. That war ended with the reintegration of Megara into the League. The two Leagues eventually came into conflict again 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 the yoke of Spartan hegemony; the Arcadians formed themselves into their own league to preserve their independence. The size of the Peloponnesian League was then further reduced by the Theban liberation of Messenia from Spartan control in 369 BC. 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 BC, many joined the Thebans or took a neutral position, 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.