The Delian League (or Athenian League) was an alliance of Greek city-states led by Athens and formed in 478 BCE to liberate eastern Greek cities from Persian rule and as a defense to possible revenge attacks from Persia following the Greek victories at Marathon, Salamis, and Plataea in the early 5th century BCE
The Delian League had a role. It's role was to keep the city-state to a make bigger empire with more soldiers to have enough to fight wars. In 431 BC, Sparta and other city-states joined forces against Athens to fight this war.