[9] The battle opened with the Spartans' mercenary peltasts (slingers, javeliniers, and/or skirmishers) attacking and driving back the Boeotian camp followers and others who were reluctant to fight.

[2] Initially, the Spartan infantry were sent into disarray when their retreating cavalry hopelessly disrupted Cleombrotus's attempt to outflank the Theban left column.

At this point the Theban left hit the Spartan right with the Sacred Band of Thebes, a troop made up of all male lovers, led by Pelopidas,[11] at its head.

[2] The normal practice of the Spartans (and, indeed, the Greeks in general) was to establish their heavily armed infantry in a solid mass, or phalanx, some eight to twelve men deep.

This was considered to allow for the best balance between depth (the pushing power it provided) and width (i.e., area of coverage of the phalanx's front battle line).

In a major break with tradition, Epaminondas massed his cavalry and a fifty-deep column of Theban infantry on his left wing,[2] and sent forward this body against the Spartan right.

[2] Wilhelm Rüstow and Hermann Köchly, writing in the 19th century, believed that Pelopidas led the Sacred Band out from the column to attack the Spartans in the flank.

The Thebans somewhat bent the rules by insisting on conditions under which the Spartans and allies recovered the dead and by erecting a permanent rather than perishable trophy – something that was scrutinized by later writers.

Further, Philip II of Macedon, who studied and lived in Thebes, was no doubt heavily influenced by the battle to develop his own, highly effective approach to tactics and armament.

Philip's victories against the Illyrians and at Chaeronea and Alexander's triumphs at the Granicus, Issus, Gaugamela, and the Hydaspes owe credit to the tactical maneuver used to vanquish the Spartans.

Historians Victor Davis Hanson and Donald Kagan have argued that Epaminondas' oblique formation was not an intentional and preconceived innovation in infantry tactics, but was rather a clever response to circumstances.

Because Epaminondas was already outnumbered, he had no choice but to form fewer companies and march them diagonally toward the much longer Spartan line in order to engage as much of it as possible.

[15] The battle is fictionalized, though in some detail, in David Gemmell's book, Lion of Macedon, which includes the significant deviation from historical canon in that it is credited to a young Parmenio(n) instead of Epaminondas, who serves merely to gain permission to carry out the echelon tactic.