At the battle of Leuctra, the Theban army used innovative new tactics to defeat a significantly larger force of Spartans, at the time supposedly the greatest warriors in Greece.
The growing power of Thebes was a threat to Sparta's pre-eminent position in Greece at the time, and so when some of the cities of Boeotia asked for assistance in breaking away from their Theban overlords, Sparta sent an army to help. This was not the first conflict between Sparta and Thebes but part of an ongoing rivalry.
The Spartan army of the day was one of the finest fighting forces in Greece. Traditionally the best elements were placed on the right of the line, but even the Spartans' left-flank units were considered a match for the best in anyone else's army. It was thus with confidence that King Cleombrotus led his army into Theban territory. Epaminondas of Thebes had no choice but to intercept the Spartans, but he faced certain defeat if he fought a traditional line battle. The only troops in the Theban army that could take on Spartans on equal terms were the 300-strong Sacred Band.
Even if he had not been outnumbered, Epaminondas would still have been in a difficult position. His solution was to concentrate his infantry in a deep and powerful phalanx on the left-wing and to refuse his right. By bending his line back and defending it with light troops and cavalry, Epaminondas hoped to prevent the Spartans from engaging the weak wing until his powerful left-wing phalanx had done its work.
A Decisive Clash
The Spartan attack on Epaminondas' refused right flank went badly. First, their cavalry were driven off by a counter-attack, throwing the line into disorder as they fled. Then the disorganized attack was beaten off by the Theban light troops and cavalry.
Meanwhile, on the Theban left flank the phalanx rolled into the Spartan line, crushing the best of the Spartan troops, including the Royal Bodyguard. King Cleombrotus was killed as the right-wing of his army disintegrated. The Thebans then wheeled to roll up the Spartan line, completing their victory. The ensuing Spartan retreat left Boeotia in Theban hands and inspired rebellions among Sparta's previous conquests. The Battle Of Salamis: A Huge Naval Battle Between A Greek Alliance & The Persian Empire In 480 BCE