The Athenian fleet then turned to chasing the Persians from the Aegean Sea, defeating their fleet decisively in the Battle of Mycale; then in 478 BC the fleet captured Byzantium. At that time Athens enrolled all the island states and some mainland ones into an alliance called the Delian League, so named because its treasury was kept on the sacred island of Delos. The Spartans, although they had taken part in the war, withdrew into isolation afterwards, allowing Athens to establish unchallenged naval and commercial power.