The Sacred Band of Thebes was a hundred and fifty pairs of lovers, organised in 378 BC by Gorgidas and reformed by Pelopidas as an elite shock unit of the Theban army. The theory, Plutarch wrote later, was that no man would behave shamefully in front of the man he loved. The unit was barracked at Theban expense, trained constantly, and maintained at full strength by the city. It was the first Greek standing professional infantry, and for forty years it was undefeated by anything its commanders pointed it at. At Tegyra it had a problem larger than itself.

By 375 BC, no Spartan formation had ever lost a set battle to a numerically inferior or equal Greek force. Spartans had been beaten by the irregular tactics of the Athenian general Iphicrates, who used light troops to wear down hoplites; they had been worn down in skirmishes; but no smaller Greek army had ever lined up against them and won. Boys in every city in Greece grew up assuming this was a fact of nature. The Spartan garrison commander at Tegyra, seeing the Theban column trapped between him and the marsh, would have made the same assumption. The numbers favoured Sparta - Plutarch puts the Spartan force at perhaps two morai, around twelve hundred men, against three hundred Thebans and a small mounted detachment. There was a confidence in marching forward that came from never having needed to be careful.

Pelopidas did not spread his line to match Spartan frontage. He concentrated. He massed the Sacred Band into a deep column and aimed it at the Spartan polemarchs - the senior officers visible by the helmet crests at the centre of the line. The cavalry hit the front. The Sacred Band hit the centre. Both Spartan polemarchs were killed in the first minute, and the formation came apart from the inside. The Sacred Band did not press their charge; they let the Spartans break and run. Spartan dead in number is not recorded; Theban dead, by the ancient accounts, were few. Pelopidas was holding what was left of the road and the Spartans were no longer on it.

Tegyra changed nothing tactical. Pelopidas did not press the advantage; the larger Spartan force inside Orchomenus would have crushed his small column if he had. He withdrew to Theban territory with his Sacred Band intact, and the Spartans regrouped to find that they had lost a small fight in a forgotten part of Boeotia. What was different was symbolic. Diodorus Siculus records it as the first Theban victory commemorated with a battlefield trophy raised over Spartan dead. For the Spartans, it was the first time in Greek memory that a Spartan formation had been broken in a pitched engagement by a smaller force. Plutarch saw the connection plainly: Tegyra was the prelude to Leuctra. Four years later, in 371 BC, a Theban army again under Theban command, again with the Sacred Band as its hammer, met a much larger Spartan army on the plain of Leuctra. Sparta lost four hundred Spartiates in an afternoon - more full citizens than the city had to spare - and the system of helot-supported full-time Spartan warriors that had dominated Greece for three centuries effectively died in that field.

Tegyra is now Polygyra, a small village in modern Boeotia between Orchomenos and Lake Kopais - except Lake Kopais is no longer there. The lake was drained by British engineers in the late nineteenth century to create farmland, and the marsh that closed off Pelopidas' flank now grows cotton and corn. The temple of Apollo at Tegyra, where Plutarch says oracles were given before the battle, is gone. What remains is the geography: the narrow strip of road between hill and former marsh, the way the line of advance from Orchomenus funnels any column into a fight with the men coming the other way. Stand on the road and you can see why Pelopidas had no choice but to fight, and why Sparta - having him exactly where they wanted him - never thought to be careful.