Peloponnesian League: modern name for the alliance of Sparta. This informal coalition of towns on the Peloponnese had its origins in the Archaic period; in the classical age, it was opposed to (and overcame) the Delian League of Athens.
The origin of the Peloponnesian League is sought in the sixth century BCE, when in many places in the Greek world long-lasting military coalitions were concluded. The alliance that was called "the Spartans and their allies" was one of them. It is not clear why and how it was created, but it is not a wild guess that Spartan aggression was a factor. The city-state that had conquered Messenia and reduced its inhabitants to serfdom (they became helots), needed to create a system of political control to guarantee that the Messenians would not receive outside support.
Still, this can not have been the only factor that forced other Peloponnesian towns into an alliance. Not unlike the creation of the nineteenth-century European empires, the growth of the Spartan coalition was caused by local factors as well: Sparta was pulled into alliances as well. One such factor may have been that the Spartans were often willing to help people expel tyrants. If this happened in far-away places like Samos or Athens (where the Spartans expelled Polycrates and Hippias), the ties of friendship were fleeting, but Peloponnesian states joined the Spartan alliance on a more permanent base (e.g., Elis and Corinth).
It is also reasonable to assume that towns that felt threatened by Argos gladly asked Sparta for protection. In the sixth century, every Peloponnesian town had to make this choice between Sparta and Argos. Herodotus of Halicarnassus suggests the fact that Tegea was forced and/or decided to join the Spartan alliance was the decisive moment. A part of the treaty is quoted in the Greek Questions by Plutarch of Chaeronea, who in turn quotes Aristotle of Stagira, and although the interpretation is notoriously difficult, it is clear that Tegea gave up some of its autonomous foreign policy in return for Spartan protection. After this, the balance of power between Argos and Sparta tipped in favor of the latter, the Spartan king Cleomenes defeated the Argives, and Sparta became the uncontested master of the Peloponnese.
The Peloponnesian League can best be seen as a network of probably bilateral perpetual alliances. Except for Argos, all towns on the Peloponnese were member of the Spartan alliance. Most of them had sworn, like Tegea, to subordinate their foreign policy to Spartan wishes and received protection in return. Sparta could also call for (perhaps one third of) a town's soldiers, who had to serve under Spartan command. Still, a major city like Corinth was left a considerable freedom. The historian Thucydides records how in the late 430s, the Corinthians waged war against the Corcyrans without much care about Spartan leadership. The Peloponnesian League was, essentially, a loose organization of towns that shared some sentimental ties, like the cult of Heracles and the believe that many of them were Dorians. Religious festivals like the Olympic Games were, although open to all Greeks, celebrations of this Dorian sentiment.
The Peloponnesian League had no permanent institutions. Most affairs were dealt with bilaterally, and the representatives of all member states only met when the Spartans wanted it. The Congress of the Allies was a rare event. It appears that every ally had one vote, but this does not mean that the leader's wishes could be overcome, because the votes of small towns, like Orneae or Phlius, appear to have been controlled by Sparta, and besides, Sparta was not forced to accept a Congressional decision.
During the Persian War (480-479), the Peloponnesian League was the model of the Hellenic League that fought against the invaders. It met at the Corinthian isthmus. It would not be exaggerated to say that all Greek nations had now more or less become members of the alliance of Sparta and accepted its leadership. This unity did not survive when the danger had passed. The Spartan leader Pausanias was incapable of keeping the allies together, so that the Hellenic League fell apart in a restored Peloponnesian League and the Delian League, which was under Athenian leadership.
The Delian League was much more advanced, being inspired by the system of the Achaemenid Empire. In the course of the fifth century, the Athenians were able to convert their alliance into a centralized empire, which many Greeks regarded as an infringement upon their liberty. The loosely organized Peloponnesian League was considered to represent liberty: what the Spartan alliance lacked as organization, it compensated by goodwill.
After the Peloponnesian War, in which the Spartans and their allies defeated the Athenians and their allies, the Peloponnesian League continued to exist, even when the Persian king Artaxerxes II Mnemon made the Greeks sign a common peace treaty in which the autonomy of all towns was guaranteed ("the King's peace", 387/386). Because the members of the Peloponnesian League were believed to voluntarily follow Sparta's lead, the autonomy clause did not apply to the Spartan alliance.
In the 370s, Sparta started to copy the Athenian system of raising tribute from the member states, which made it clear to all that Sparta was an imperial power like any other. In 371, it paid the price. After the battle of Leuctra, in which the Thebans defeated the Spartans, few Greeks were willing to fight for the survival of the Peloponnesian League. It disintegrated in the following years and was dimantled in 365.