Alongside the social differences, Sparta had disapproved of Athens' use of money from the Delian League, since Athens had used the money in attempt to protect the city states, for its own benefit. When the city states protested against the inequality and tried to free themselves from the Athenian's rule, Pericles punished them. Having enough of the Athenian aggression, Sparta declared war on Athens.
While Athens had a strong navy, Sparta had a strong army. With two different strengths, the two city-states had different strategies for winning the war. Athens, under rule from Pericles, chose to avoid fighting on land and hope to win the war on sea. Athens stayed within it's city walls and allowed ships to keep the city supplied.
430 B.C., Athens had been suffering from a deadly plague. It was most likely from the result of a highly infectious disease, typhoid disease. With such crowded streets, the fever had rapidly turned into a deadly plague. In just four years, the disease had killed about 60000 people, which was doing much more than the Spartans.
421 B.C., the Athenians and Spartans had signed a truce. the truce was intended to last 50 years, yet it ended 48 years short. Athens and Sparta had been pulled back into war over the lands of Sicily. A dispute between the two city-states had prompted Athens to invade. The Athenians had decided to invade Syracuse, however the invasion was disorganized. The Spartans were able to call reinforcements, and the Athenians were defeated.