Epaminondas kicking ass. Illustration by W. Rainey.
Back in Greece, there was one city state, a militaristic slave society, hated and feared above all others. At the start of the 4th century B.C., Sparta ran the neighborhood, setting up and toppling governments, generally acting like bastards to everyone else.
Their main rival broken, they seemed invincible. The newly democratic Thebes, one of the only cities left standing with a chance of stopping them, especially hated them.
Despite the torrent of crap about their merits in recent years, especially all fictions about the stand of the 300, the Spartans were hands-down the villains of this era. In Thebes' case they'd once been allies, but the Spartans betrayed them, took over their capital and put in a government they despised, propped up by occupying soldiers.
A few years later, the Thebans took back their city and reasserted their rights. Sure enough, the second they start gaining serious power again, the Spartans send out an army to remind them who's boss.
So in 371 B.C., the Thebans turned to Epaminondas, a now nigh-forgotten leader who had some experience as a soldier but more of a reputation for philosophy and integrity. But sometimes the best people for war aren't warriors; this was one of those times.
Forget Thermopylae for a moment, because this story is way more relevant, and more interesting. Epaminondas was about to do the impossible: break Sparta.