Before the Peloponnesian Wars, Athena was the most powerful polis and had recently created the Delian League. Sparta started to get jealous and jealousy was one of the big factors of the cause of the war.
Thanks so much for inviting me here! No Sparta?
Hello! Welcome to the Delian League!
Fine! We don't care if we aren't in your ally club, we'll just create our own! Better yet, we'll destroy you!
During the wars, Athens was sort of locked into the city and couldn't go out of the city to get food/crops. Because of their strong navy, they were able to get food to the city, and avert the Spartan army on land and outside the city.
Go Athenian Navy!
Yes, finally! The food is here! No more hunger!
After the Peloponnesian Wars, the Greek city states were all divided. They were no longer a united nation together, but all separate. Because of this, Alexander the Great and Phillip the Second were able to conquer Greece.
We don't want to talk to you! Leave our city state!
Hey, we don't want to be a united country! Leave Athens alone, we have to regrow all of our crops!
Sparta and Athens are the problem, not us! They caused the division, and now we're getting conquered!