"The Delian League was turning from an alliance into an empire" (source: Wikipedia)
Could have led instead to a United States of Greece in the 5th century BC.
Had that happened, history, at least European one, would have been quite different.
Can Europeans and their states, via the EU, form the federal state that the Greek city states were unable to do? What difference can this make to the future of Europe and the world (Earth)?
Failure? The Greeks were occupied by the Romans, the Byzentines and the Ottomans, forming a state only in 1831, 2200+ years later!
Will a United European state take 2200+ years, ie will Europeans have to wait until 4212 and beyond? And who are going to be the Romans, Byzantines, et al in Europe's case?