http://www.britannica.com/EBchecked/topic/156539/Delian-League Encyclopædia Britannica: Delian League (Citation: While the Athenians and allies were campaigning successfully against the Spartans, subjugating Aegina, Boeotia, and central Greece, further expansion was checked when the league fleet was virtually destroyed in Egypt. Fearing the Persians would mount an offensive following such a naval defeat, the Athenians transferred the league treasury to Athens (454). Within the next five years, with the resolution of difficulties with Sparta (five-year truce, 451) and Persia (Peace of Callias, c. 449/448), the league became an acknowledged Athenian empire.)