The Peloponnesian war: Athens becomes very wealthy, powerful and they had the stronger navy out of the 2 cities, Athens and Sparta. Sparta was located in the inland and couldn't fight on water. Sparta was jealous of the other powers and desired more power for itself. Sparta was also unhappy about not having all the military glory.
That's the plague
Spartans were so mad at Athens and decided to march into Athens to burn all of the food. The Spartans were so mad because the Athenians thought they were better than Sparta which made them mad and caused the marching into Athens.
As soon as the Spartans started marching into Athens the Athenians all gathered into the walled city. The walled city was a full city, it was just protected from the Spartans. And stopped outside activities the Athenians did.
After 2 years of the Athenians stuck inside the walled city a plague struck. A plague struck right inside Athens killing 1/3 of the population and Pericles.
After the plague struck inside Athens, they created a truce. Peace didn't last long, 415 BCE Athenians sent a huge fleet of 20,000 men to the island of Sicily. Athens planned to destroy one of Sparta's wealthiest allies, and ended with a crushing defeat.
After the war, Athenians lost confidence in democratic government and began to question their values.