How were the Thebans able to conquer the Spartans after the Peloponnesian War?

Decisive battle

http://en.wikipedia.org/wiki/Image:Leuctra.png

Top: Traditional hoplite order of battle and advance.

Bottom: Epaminondas's strategy at Leuctra. The strong left wing advanced while the weak right wing retreated. The red blocks show the placement of the elite troops within each phalanx.

The fighting opened with a cavalry encounter, in which the Thebans were victorious. The Spartan cavalry was driven back into the ranks of the phalanx, disrupting the order of the infantry. Seizing the advantage, the Boeotians pressed the attack. Cleombrotus was killed, and although the Spartans held on for long enough to rescue his body, their line was soon broken by the sheer force of the Theban assault. At a critical juncture, Pelopidas led the Sacred Band in an all-out assault, and the Spartans were soon forced to flee. The Peloponnesian allies, seeing the Spartans put to flight, also broke and ran, and the entire army retreated in disarray. Four thousand Peloponnesians were killed, while the Boeotians lost only 300 men. Most importantly, 400 of the 700 Spartiates on the scene were killed, a catastrophic loss that posed a serious threat to Sparta's future war-making abilities.