The conditions which led to the second Athenian or Delian Confederacy were fundamentally different, not only in virtue of the fact that the allies had learned from experience the dangers to which such a league was liable, but because the enemy was no longer an oriental power of whose future action there could be no certain anticipation, but Sparta, whose ambitious projects since the fall of Athens had shown that there could be no safety for the smaller states save in combination.
The details of his conquests are uncertain, but it is known that in the Cyclades he maintained an alliance with the tyrant Lygdamis of Naxos, and curried favour with the Delian Apollo by dedicating to, him the island of Rheneia.
It became known as the "Delian problem" or the "problem of the duplication of the cube," and ranks in historical importance with the problems of "trisecting an angle" and "squaring the circle."
After the defeat of Xerxes the Thasians joined the Delian confederacy; but afterwards, on account of a difference about the mines and marts on the mainland, they revolted.
Halicarnassus and other Dorian cities of Asia were to some extent absorbed by the Delian League, but the peace of Antalcidas in 387 made them subservient to Persia; and it was under Mausolus, a Persian satrap who assumed independent authority, that Halicarnassus attained its highest prosperity.