History of the Peloponnesian War

When those of the Argives who heard their proposals had reported them to the government and the people, the Argives passed the decree, and chose twelve men, with whom any one of the Greeks who wished should conclude an alliance, except the Athenians and Lacedaemonians, neither of whom should have liberty to enter into treaty without the consent of the Argive people.

The Argives acceded the more readily to these proposals, because they saw that they should have the war with the Lacedaemonians, (for their treaty with them was on the point of expiring,) and also because they hoped to gain the supremacy of the Peloponnese. For at that time Lacedaemon was in very bad repute, and was despised in consequence of its misfortunes; while the Argives were in an excellent condition in all respects, as they had taken no part in the war against Athens, but had rather reaped the good fruits of having been in treaty with both sides.

Thus, then, the Argives were admitting into alliance such of the Greeks as wished it.