How did the war lead to the ultimate decline of the Classical Greek period and the city-states of Athens and Sparta.

Must use The Peloponnesian War by Donald Kagan (I HAVE THE PDF)
Your paper should use at least 5 sources total, including both primary & secondary sources. Approximately 50% of your citations should be from the monograph, while the other 50% of your citations should be from other sources.

QUESTIONS:

PELOPONNESIAN WAR:
How did the war lead to the ultimate decline of the Classical Greek period and the city-states of Athens and Sparta.
JUSTINIAN’S FLEA:
How did the plague ultimately weaken the organization of Rome and how did this lead to a division of East and West?