Successful Classical Leagues

During Antiquity, there were any number of Leagues of city-states: Delian, Achaean, Aetolian, Ionian, Latin, and others.

What circumstances could lead any of these leagues to survive for an extended period and prosper? Admittedly, one could argue that the Latin league did indeed do just that, but only insofar as its members were absolutely subordinate to Rome and divested of any real political authority, even though their constituents were eventually granted political rights within the Roman system itself.

So, a situation in which such a league survives and maintains some legal parity between its various members, without one coming to totally dominate every other, would be ideal.
 
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