The main aim of the Delian League was to maintain the freedom of Greece from Persia. The League was set in place to organise and defensive alliance so that in any future attacks upon their territory, the Greeks would be well prepared to resist.
Method 1
The revolt and subjugation of Naxos in 469 was one of the first circumstances where a League ally lost its autonomy. Naxos became a subject, a tribute-paying state, and it became apparent that in the future, Athens, as the leader of the League, would not hesitate to use force to make sure that obligations were fully met.
Method 2
An important turning point in Athens’ relations with her allies was the grow of radical democracy. (ie Cimon)
Method 3
In 454BC, the allied treasury at Delos was moved to Athens on the excuse of a possible Phoenician raid into the Aegean. Although reasons of safety could be justified, the move was a significant step in the transformation of the League into an Empire.
Method 4
Under the influence of Pericles, an avowed imperialist, the allies of the League were required to support Athens in pursuing her own interests during the years 459-454BC. (Talk about the naval policy, and how Pericles persuaded the subjects to give money NOT for war ships but for his building program)
Method 5
A method of control used by the Athenians was to put garrisons in rebellious cities. These Athenian garrisons not only served a military purpose but were a political device as well.
Method 6
Between 450 and 446BC, Athens inaugurated a system of cleruchies, which were settlements of Athenian citizens abroad. These strengthened Athens’ hold on her empire, as they were located at strategic points in the Aegean.
Method 7
The revolt of Euboea and Megara & the treatement of Samos, and Periclean power in these subject states
Method 8
Membership of the empire provided advantages such as peace, better protection, democratic governments, and prosperity through increased trade. However the loss of the member’s independence in domestic and foreign affairs, their economic dependence on Athens and her high-handed actions towards the allies far outweighed any advantages gained. They saw her becoming a tyrant state.
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I hope this helps. Good luck!