Sparta aquired 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 leage provided protection and security to its members, and most importantly to Sparta. It was a very stable alliance, and supported Oligarchies and opposed tyrannies.