Answer
Sparta, the formidable Greek city-state, faced a formidable challenge from Epaminondas, the brilliant general of Thebes. Epaminondas, known for his innovative military tactics, led Thebes to victory over Sparta at the Battle of Leuctra in 371 BC, breaking Sparta's dominance over Greece. This challenge marked a significant shift in the balance of power in ancient Greece, as Thebes emerged as a major force, challenging Sparta's long-held supremacy.