Hey there!
The Peloponnesian war was a war fought between the Delian League, led by Athens, and the Peloponnesian League, led by Sparta.
It was different from other conflicts like the Persian Wars because there was no outside conflict. It was a conflict between the two Greek "Superpowers" and they both felt like it was necessary to gain an edge over the other.
After fighting this war, Sparta was victorious. However, it left the entirety of Greece weak. Philip II of Macedonia took advantage of this, and conquered Greece.
Hope this helps!