Why was the Battle of Leuctra so important?

A surprise defeat to the professionally trained Spartan army, Leuctra heralded an end to the Spartan state as a dominant force in Greek politics. The city-state of Thebes dismantled the Spartan military so that it no longer posed a military threat. This source-based homework task is designed to support the NSW Ancient History Syllabus Higher School Certificate course
Part II Ancient Societies
Option I - Spartan Society to the Battle of Leuctra.
It can be used without modification to support the Queensland Ancient History Syllabus Theme 5 Studies of Power - The Military Elite in Sparta.