After the Athenian defeat in the Peloponnesian War, Sparta was the leading power in Greece. Immediately, it started to support the revolt of Cyrus the Younger against the Persian king Artaxerxes II Mnemon, and later, the Spartan king Agesilaus invaded Asia. The only result was that the Persians started to support Athens, which returned to its former power in 395.
To keep the Greeks divided, the Persians continued to switch sides. They were responsible for the rise of Thebes, supporting general Epaminondas, who had already showed his qualities by improving the phalanx tactic. Perhaps Thebes would have become a great power without Persian support, but the money accelerated the Theban expansion..
In the summer of 371, the Spartan king Cleombrotus, Agesilaus' younger colleague, invaded Boeotia with a large army that was to settle all accounts. At Leuctra, it met the Theban army of Epaminondas, which was perhaps half the size of the Spartan army.
However, Epaminondas placed his troops at an angle with the Spartan troops, and was able to concentrate his forces on one section of the Spartan battle line. The oblique Theban phlanx broke through the Spartan lines, and their victory was complete. For the first time, the Spartans had been defeated by an army smaller than their own. Even worse, it had hardly any soldiers left, and the next decades it was to look for money to buy mercenaries.
Epaminondas invaded the Peloponnese, liberated the helots of Messenia, and repeated his success at Mantinea (362). Although he was killed during the second battle, his inheritance was important enough: Sparta was no longer a great power.