The League was the earliest and the longest lasting of all known Greek alliances. Despite its name it included all of Sparta's allies and not only Peloponnesian cities.
Citation:
Bouchard, Elsa. "Formation of the Peloponnesian League".
The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11179, accessed 23 May 2013.]