It is usually taken for granted that throughout the first part of the fifth century B.C. until her defeat by Athens in or about 457 Aigina was a member of the Peloponnesian League and was consistently hostile to Athens and to the formation and growth of the Confederacy of Delos. I believe that the evidence for this view is weak, and that Aigina was never a member of the Peloponnesian League but probably was a voluntary member of the Delian League from its formation.
About 494 King Kleomenes of Sparta invaded the Argolid and defeated the Argives at Sepeia. For this invasion he used some ships belonging to Aigina, which he had taken by force. This implies that some kind of fight must have taken place recently between Aigina and the Spartans, or their allies. How could the Spartans, who were a land and not a sea power, capture ships from the Aiginetans, who had one of the largest fleets in Greece at that time? Only with the help of an ally whose naval power was stronger than Sparta's. Obviously this ally must have been Corinth. Corinth was always a naval power, and at this period she was hostile to Aigina, and she was undoubtedly a member of the Peloponnesian League. So the forces of the Peloponnesian League fought and defeated Aigina shortly before they defeated Argos in the middle 490s. There were, of course, traditional connexions between Argos and Aigina. Argos had assisted Aigina in a war against Epidauros and Athens long before; while after her defeat at Sepeia Argos demanded a fine of 500 talents from Aigina as an atonement for her treachery, and she could not have made this demand if she had not had, or claimed to have, some kind of alliance with her. It is therefore not surprising that the Peloponnesians should attack both Argos and Aigina at the same period.