This alliance, made up mostly of city-states on the Peloponnese, the southernmost peninsula of the Greek mainland, was much less formal than the Delian League. Athens was part of the Delian League, an alliance of ancient Greek-city states led and funded mainly by Athens that eventually morphed into the Athenian Empire, and Sparta was a member of the Peloponnesian League. However, rarely did the two sides fight each other alone. The Peloponnesian War was fought mainly between Athens and Sparta. But before going into all the details, here are the main points to remember: Who Fought in the Peloponnesian War? The Peloponnesian war lasted 27 years, and it occurred for many different reasons.