Sparta
The Greek city-state that turned its whole society into an army, sustained by mass slavery, feared for Thermopylae and broken at Leuctra.
Sparta did not have an army. Sparta was an army with a city attached. Alone among Greek states, it turned its entire society into a machine for producing hoplites, and to feed that machine it enslaved an entire neighboring people. The result was the most feared land force in the Greek world and one of the most rigid social systems humans have ever built.
Bred for the phalanx
A full Spartan citizen, a Spartiate, existed to fight. At age seven the state took boys from their families into the agoge, a public upbringing designed to break softness out of them: minimal food, one cloak a year, beatings, and encouragement to steal to survive, where the crime was getting caught rather than stealing. Sickly newborns, Plutarch reports, were inspected by elders and left to die on the slopes of Mount Taygetus. Men lived in barracks until thirty and ate at communal mess halls for life. Everything private, comfort, wealth, individuality, was treated as a threat to the collective. The word 'spartan' still means bare to the point of harshness, and that is exactly what the Spartans intended.
The helots
This full-time soldiering rested on slavery at an industrial scale. When Sparta conquered neighboring Messenia in the 8th and 7th centuries BC, it reduced the population to helots, state-owned serfs who farmed the land so Spartiates never had to work. Helots outnumbered their masters by perhaps seven to one, and Sparta governed them by terror. Each year the state formally declared war on the helots so that killing one carried no religious pollution, and the krypteia, a secret cadre of young Spartiates, murdered any who seemed strong or defiant. A society this dependent on the people it brutalized lived in permanent fear of revolt, which shaped nearly every decision Sparta made.
Prestige and rot
Two events fixed Sparta's reputation. In 480 BC King Leonidas and his 300 died holding the pass at Thermopylae against Persia, buying Greece time and buying themselves immortality. In 404 BC Sparta finally beat Athens to end the 27-year Peloponnesian War, briefly becoming the dominant power in Greece.
Then the system ate itself. Spartiate status required land and a fixed contribution to the mess, and battlefield deaths, earthquakes, and concentrated wealth steadily shrank the citizen body, a crisis the Greeks called oliganthropia. The roughly 8,000 Spartiates of 480 BC had fallen to perhaps 1,000 by the 370s. In 371 BC the Theban general Epaminondas shattered the Spartan phalanx at Leuctra, killing about 400 citizens in a single afternoon, then marched south and freed Messenia. Sparta lost its slaves, its farmland, and its myth of invincibility at a stroke. It never recovered, and by 192 BC it had been forced into the Achaean League, its ancient Lycurgan system soon abolished altogether.
The myth versus the machine
What survives in popular memory is the heroic Spartan: the laconic warrior, the last stand, the abs of the film '300.' The real Sparta was grimmer and stranger. It practiced eugenic infanticide and ran a police state over an enslaved majority, yet it also gave women more freedom than anywhere else in Greece, letting them own property, exercise in public, and speak their minds, because healthy mothers produced strong soldiers. Sparta was not a nation of free heroes. It was a totalitarian experiment that traded almost everything human for the ability to win one kind of battle, and when it finally lost that battle it had nothing left.