The Thebans. Their Sacred Band was one of the premiere fighting groups in all of Greece. Thebes was a bitter enemy of Athens for most of its history, but at times would be at odds with Sparta. Shockingly, in 371 BC, the Thebans, led by the great General Epaminondas, defeated the Spartans at the Battle of Leuktra. This would usher in the decline of Spartan power and prestige.
Epaminondas, was a Theban general and statesman of the century BC who transformed the Ancient Greek city-state of Thebes, leading it out of Spartan subjugation into a preeminent position in Greek politics.