Athens lose the Peloponnesian war.

By all calculations Sparta should have lost the war. Athenians had strong leadership and the money to finance a war. Their Navy ruled the seas and was virtually unbeatable on the seas. Sparta had little money, weak leadership, and no navy. Their only true strength was a strong ground force. Sparta is able to win the war by obtaining Persian monies as support, a strong leader (Lysander), and reaping the rewards of the Athenians mishaps. …

…and Nicias. The Athenians own internal conflicts and democracy prevented them from maintaining a sound policy on all aspects of their empire. In the end the Spartans won the war because the Athenians arrogance, error in judgment, and the Persians assistance. The Spartans with the aid of Lysander and Cyrus was finally able to stabilize their policies during a time Athens were under political unrest. With these factors the Athenians lost their empire to Sparta.