Thanks in part to the battle of Thermopylae in 480 B.C., in which a small force of Spartan soldiers stayed behind to fight to the death against a vastly larger Persian army, the warriors of Sparta have long been famous for their military prowess and tenacity. Even today, the word “Spartan” conjures up an image of an awesomely fit, skillful fighter, indifferent to pain and fear.

“Other [Greek] city states had fine armies,” explains Kimberly D. Reiter, an associate professor of ancient and medieval history at Stetson University. “Sparta was recognized by most as the best.”

How did the Spartans become so awe-inspiring? One factor was the agoge, the Greek city-state’s educational and training system, which used harsh, extreme and sometimes cruel methods to prepare boys to be Spartan citizens and soldiers.

“The agoge aimed to instill soldierly virtues: strength, endurance, solidarity,” as the late Canadian historian Mark Golden wrote. But it accomplished all that at great cost, by turning Spartan boys’ childhood into what today would be seen as a traumatic experience.

According to the ancient Greek historian Plutarch, who wrote several centuries after Sparta’s heyday in the 400s B.C., Spartans began developing soldiers shortly after birth, when male infants were evaluated by Spartan elders. The “well-built and sturdy” children were allowed to live, while those who were deemed unhealthy or deformed were left at the foot of a mountain to die.

At age seven, Spartan boys were turned over by their parents to the state, where they were organized into companies that lived, studied and trained together.

“The boy who excelled in judgement and was most courageous in fighting was made captain of the company,” Plutarch wrote. “The rest all kept their eyes on him, obeying his orders and submitting to his punishments, so their boyish training was a practice of obedience.”

Plutarch portrayed Spartan boys as receiving little schooling. But Stephen Hodkinson, an professor emeritus of ancient history at the University of Nottingham, UK, says there are hints in other sources that they received “the standard Greek elementary education in reading, writing, numbers, song and dance.”

To toughen them up even more, Spartan boys were compelled to go barefoot and seldom bathed or used ointments, so that their skin became hard and dry, Plutarch wrote. For clothing, they were given just one cloak to wear year-round, to make them learn to endure heat and cold, and made their own beds from plants that they had to rip out of the ground with their bare hands from river banks.

According to Plutarch, as the young Spartans grew, they were required to exercise more and more to build their bodies. As Donald G. Kyle notes in his book Sport and Spectacle in the Ancient World, Spartan youth had to present themselves for regular inspections in the nude, and boys who didn’t look sufficiently fit were flogged.

In addition to foot races and wrestling, their sports included a particularly brutal contest in which two teams would try to drive each other off an island by pushing, kicking, biting and gouging their opponents, according to Kyle’s book.

To make life even tougher, Spartan boys were fed a meager diet. Xenophon, a philosopher and historian who lived from the late 400s to mid-300s B.C., noted that one purpose was to keep them slim, which Lycurgus, the founder of the Spartan system, believed would make them grow taller. But the boys’ hunger was also intended to embolden them to steal food from gardens and other places “in order to make the boys more resourceful in getting supplies, and better fighting men,” Xenophon wrote. But to make sure they learned cunning, boys who were caught stealing were whipped.

Such harsh punishment was a prominent part of the Spartan training system. The Spartans even turned it into an annual ritual, in which boys tried to steal cheeses from a temple altar, which required them to evade guards armed with whips.

“Whipping was a test of courage and stoicism,” Reiter says. “Boys looked forward to the public display of their fortitude.”

The Agoge was a “trial by ordeal,” as Paul Cartledge, a professor emeritus of Greek culture at the University of Cambridge, wrote in his 2003 book Spartan Reflections. But it was a vital step toward being selected for one of the messes, the communal dining groups, and becoming a full-fledged Spartan citizen and soldier.

Strictly speaking, the Agoge didn’t include military training, which didn’t start in earnest until they became adult soldiers. Its real focus was to prepare Spartan males to be compliant members of society, who were ready to sacrifice their all for Sparta. Unlike other Greek city-states, Sparta “was exceptional in its socio-political stability,” Hodkinson says. “Part of the reason for this was that the boys’ upbringing had instilled behaviors that encouraged harmony and cooperation.”

But Spartan schooling’s emphasis on fitness did help Spartan soldiers on the battlefield. “It made them tougher/stronger, more able to sustain the weight of a heavy basically wooden shield in the summer sun, better at pushing and shoving, better at stamina,” Cartledge says.

The Spartans’ real secret wasn’t physical fitness or indifference to pain and suffering, but rather superior organization. Spartan troops drilled relentlessly, until they could execute tactics with perfection. “It was probably their training in tactical maneuvers which really gave Spartan soldiers their edge on the battlefield,” J.F. Lazenby writes in his book The Spartan Army.

“Xenophon says a Spartan army could perform maneuvers that others couldn’t, because of their training,” Cartledge says.

According to Plutarch, Spartans continued regular military training throughout their adult lives. “No man was allowed to live as he pleased, but in their city, as in a military encampment, they always had a prescribed regimen,” he wrote. As Cartledge writes in Spartan Reflections, it wasn’t until age 60 that Spartans finally were allowed to retire from the army—provided that they lived that long.

The stability that the agoge fostered also “led to a certain inflexibility,” Hodkinson says. For all the Spartans’ efficiency, they relied heavily on a limited set of maneuvers, and when those failed, they didn’t have a plan B.

Off the battlefield, the rigid acceptance of the status quo that the Spartan educational system enforced made it difficult for the Spartans to deal with social problems in their society, such as inequality in land ownership and a declining population.

“Eventually it produced a sort of conceptual lock when Spartans could not imagine any other kind of life,” Reiter explains. “This made it very difficult for Spartans to accept innovation in war or politics.”

In that sense, the regimen that helped make the Spartans so tough also contributed to Sparta’s ultimate downfall. In 371 B.C., Thebes, a rival city state, defeated Sparta at the battle of Leuctra by using unorthodox, creative cavalry maneuvers that the Spartans were too inflexible to counter. That ended Sparta’s military dominance, though their fearsome reputation lived on through history.