The Peoloponnesian War began because Sparta resented Athens's use of money from the Delian League. Athens used the money to protect all the city-states for its own benefit. If the city-states fought back and tried to free themselves, Pericles would punish them. By 431 B.C., Sparta had enough of Athens's agression.
Sparta had a strong army that marched on land.
Athens had a strong navy taht fought at sea.
Athens planned to attack Sparta at sea, and withdrew inside their city walls. The Spartans marched into battle expecting a big fight. They only came to see the Athenians cowering inside their walls. Frustrated, Sparta destroyed Athens's crops in an attempt to weaken their economy. The Athenians could not prevent a devastating attack.
In 430 B.C., AThenians were hit with a terrible plague that killed about 60,000 people. They suffered from headaches, rashes, vomiting, and fever. This was an outbreak of the typhoid fever. Athens was overcrowded, so the disease spread quickly. This was taking out more Athenians than the Spartans were.
The brutal battle kept going, and Athens kept attacking at sea, and Sparta on land. The war seemed unwinnable. But finally, Athens decided on a truce.