Teaching Your Enemies

Lyrcurgus, lawgiver of the Spartans, gave the advice to his people that they ought not to make war too often against the same people, in order that no one else would learn how to wage war as well as the Spartans did.  For centuries, the Spartans were the sole professionals of war in a field of amateurs, and it was by facing them in war so often that the Thebans learnt how to defeat them at Leuctra.