we will win the war if we keep inside. Do not worry
After defeat of Persians, Greek World divided to two. Athenian Empire and its supporter (Delian Leauge) and Sparta and its supporters (Peloponnesian Leauge).
DISEASE BREAKS OUT
Athens and Spartans had very different communities and government systems. They were very intolerant to each other. Also Sparta feared if Athenians be very powerful. That eventually leaded to a outbreak of war.
Every group was thinking that they were winning at first. Athenians trusted their city's huge walls, and chose to stay inside of the walls. But Spartans were expecting and army of Athenians, so they surrounded the city. Leader, Pericles knew that Spartans had stronger army. So, he insisted staying inside.
FALL OF ATHENS
A plauge killed more than 1/3 of people in Athens. This caused war not to end for about 30 more years.
In 405 B.C. Athenian troops were defeated in Aegaspotami. In next year, Athens was surrounded by Spartans.
Spartans had won the war, the walls were collapsed. Athenian Empire was destroyed.