Athens was once in power of the delian league but had such great fear of losing power to Sparta that they made a treaty with Persia which was not popular with all of the Delian League. This caused Athens to leave the Delian League and join the Pelopennisian League. During the Pelopennesian War, Persia reneged on their alliance with Athens and went on to help Sparta create the navy that went on to defeat Athens. This led to Sparta taking complete control of Athens,, which is how they fell from dominance.
