Epaminondas is ready with a tactical innovation in the Battle of Leuctra in 371 BCE, twenty days after the peace conference.

Instead of the usual advances of heavily armed infantry drawn up in an equal number of ranks over the whole front, he masses his troops on the left wing, in a further development of the Delium arrangement of 424 BCE, to the unprecedented depth of fifty ranks against an overall Spartan depth of twelve.

This provides a flexible “tail,” or reserve force on the left that can be deployed as the course of the battle suggests.

The decision about whether, when, and how to deploy it will be the general's, whose influence on the outcome of the battle is thus greater than had been usual hitherto.

By placing the best troops on the left, the Thebans aim to knock out the best Spartan troops, who are positioned opposite them, occupying the right wing in the traditional hoplite manner.

Finally, by marching forward obliquely (rather than straightforwardly, as is customary), the Thebans increase the punch administered by this deepened left.

The novelty consists in striking the enemy first at their strongest, instead of their weakest, point, with such crushing force that the attack is irresistible.

The force of the Theban advance overwhelms the Spartans.

The Spartans lose about one thousand men, four hundred of them full Spartan citizens.

The defeat, in which democrats from Argos take part, inflicts such heavy losses on the very limited numbers of the Spartan soldiers that it seriously threatens the possibility of raising another Spartan army.

The Boeotian federation is thus saved.