What were the Spartans advantage in war?

Answer 1

Being allotted seven serfs each to support them, the Spartan warriors were able to devote themselves to training - the equivalent of a modern regular army, while other cities got by on part-time militias, their farmers getting a training day each couple of weeks in between ploughing their fields. The Spartans also, to reduce the threat of serf uprisings when they were away at war, each took seven serfs as light infantry with them, so a Spartan army of 5,000 armoured warriors numbered 40,000, as opposed to the other cities taking one light infantryman per armoured warrior = 10,000.

However the wars they were involved in progressively reduced their pool of warriors, and eventually after a series of serious battles their numbers so dwindled that they could no longer field an effective army after their defeat by Thebes at Leuctra in 371 BCE.