At the beginning of the pentekontaetia (479-431 BCE), Athens assumed control of a loose alliance of Greek poleis known as the Delian League. By the end of it, she was a hegemon over an empire of subject states, and the Delian League was a thin veil for what was really an Athenian Empire. The transformation of the Delian League into the Athenian Empire has been frequently studied by scholars, and many interpretations have come forward, explaining the events and causes of the transformation. Some believe that Athens was primarily interested in protecting the Aegean from the possibility of a Persian attack, similar to the events of 490 and 480. Others argue that Athens had visions of an empire from the very beginning of the Delian League, and possibly even earlier. Although many interpretations exist along this continuum, most would agree that by 450 Athens was acting in her own interests and had assumed the role of ruler over an empire of subject poleis.
By 454, the Delian League could be fairly characterized as an Athenian Empire; at the start of the Peloponnesian War, only Chios and Lesbos were left to contribute ships, and these states were by now far too weak to secede without support. Lesbos tried to revolt first, and failed completely. Chios, the greatest and most powerful of the original members of the Delian League save Athens, was the last to revolt, and in the aftermath of the Syracusan Expedition enjoyed a success of several years, inspiring all of Ionia to revolt. Athens was eventually still able to suppress these revolts.
To further strengthen Athens' grip on its empire, Pericles in 450 began a policy of establishing cleruchiai— quasi-colonies that remained tied to Athens and which served as garrisons to maintain control of the League's vast territory. Furthermore, Pericles employed a number of offices to maintain Athens' empire: proxenoi, who fostered good relations between Athens and League members; episkopoi and archontes, who oversaw the collection of tribute; and hellenotamiai, who received the tribute on Athens' behalf.
Athens's empires were not very stable, and after only 27 years of war the Spartans, aided by the Persians and internal strife, were able to defeat it. However, it did not remain defeated long. The Second Athenian Empire, a maritime self-defense league, was founded in 377 BC and was led by Athens; but Athens would never recover the full extent of her power, and her enemies were now far stronger and more varied.
While freeing the southern Aegean from Persian control, Cimon succeeded in convincing more states to join the league. According to Diodorus, having persuaded "the cities of the sea coast [and] the cities of Lycia" to revolt, he "took them over in the same way". This, along with further evidence, suggests that these cities were forced to join whether or not they wanted to. Plutarch tells us "Phaselis ... refused to admit [Cimon's] fleet or to fight against the King, and so he devastated their land." In 480 B.C. the Athenians attempted to force impoverished Andros to pay a sum to the league:
"the Greeks ... surrounded Andros with a view to capturing it. Andros was the first island to reject Themistokles' requests for money."
(Herodotus VIII.111)
Further evidence of expansionistic Athenian policy can be seen in the case of Carystus, the one city in Euboea which declined membership of the Delian League. After refusing to join a second time in 472 B.C., they were paid a visit by the league's fleet and promptly conquered. In both these cases the Athenian's actions were at least partially justifiable. The Athenians had secured Greek control of the Aegean and Carian. At the time of the Carystian incident the Persians still controlled these regions, and thus Carystus could become a stepping stone to mainland Greece and encourage other Euboean cities to leave the league.

A less justifiable incident was the way the Athenians dealt with Naxos' attempted secession from the league. The