http://www.idbsu.edu/courses/hy101/peloponn/02.htmThe Peloponnesian War - Page 2 of 17
Membership in the League was no longer optional, and increasingly Athens demanded money payments rather than contributions in ships. You will find some books that refer to the Athenian Empire, but such a thing never existed. Rather, Athens used the League in a manner reminiscent of modern imperialism.
Land Empire
Athenian
expansion, mostly by treaty, on land rouses Corinth to enmity. In 457 Athens
invaded and conquered most of Boeotia. In 449 Pericles transferred the
Delian treasury to the Temple of Athena, marking the changing from the
Delian League to the Athenian Empire - supposedly because the peace with
Persia made the protection of Delos unnecessary.