Sparta was a incredibly brutal army masquerading as a state. Like all great powers they were successful until they weren't.
It's weird how we forget Athenian brutality in the whole Athens/Sparta dichotomy. There Melian dialogue is literally them shaking down their neighbors and then brutally murdering them drug cartel style, and Sparta won the Peloponnesian war by being more diplomatic. We uncritically buy into both of their propaganda about themselves