Spartan Soldiers are considered to be some of the greatest fighters ever. Every Spartan boy was trained in the army. When a Spartan child was born he or she would be examined, and if it was a strong and healthy boy he would grow up to be a soldier, and if it was crippled it would be left on the mountainside to die.
Each spartan boy would be given a mentor, who would act as his guardian teaching Spartan customs and expectations. If a Spartan boy was caught stealing he would be beaten severely, not because he had stolen but because he was caught. The boys started their training at age seven and served in the army most of their life. For years they were separated from their families and lived only with other boys or men. It was considered an honor to die in battle. It was also a disgrace to lose a battle and come home still alive. A famous Spartan quote that mothers would tell their sons before battle was "Come home with your shield or on it". A fighter couldn't flee if he still kept his shield, so there was no retreat. On it would mean you died in battle.
The Spartans could afford to be good soldiers; in fact it was a necessity. They had many slaves and a peasant class who outnumbered the Spartans themselves by a wide margin. It was only through having a regimented military that they could keep their subject peoples in check.