Why was the Peloponnesian War an important war in Hellenic civilization?

It lasted 27 years, devastated the Greek world from Sicily to Asia Minor, and Athens was stripped of the alliance of Greek cities it had built up to oppose Persian power, but converted to an empire for its own benefit.

This and the subsequent fighting between the Greek cities so weakened the main Greek cities that it paved the way first for Persia to reclaim its control of the Greek cities in Asia Minor, and then for the rise of Macedonia to dominate mainland Greece and use this as a platform to move on to taking over the Persian Empire and spread a veneer of Hellenic culture through western Asia.