The correct answer to this open question is the following.
The Peloponnesian war between Athens and Sparta stretched on for nearly three decades. the Delian League, led by Athens lost the war. After the war, the city-states experienced a period of economic decline and political upheaval.
The Peloponnesian War started in 431 and ended in 404 BC with a sound victory of the Peloponnesian League led by the Spartans over the Athenians and the Delian League. After so many years of dominion in the region by Athens, after the loss, the Spartans became the most dominant city-state in Greece. The Spartans were known to be fierce warriors that trained their sons since the early years of childhood to become expert warriors.