The Spartan Army was the military force of Sparta, one of the leading city-states of ancient Greece. The army stood at the centre of the Spartan state, whose citizens' primary obligation was to be good soldiers. Subject to military drill from infancy, the Spartans were one of the most feared military forces in world history. At Sparta's heyday in the 6th to 4th centuries BC, it was commonly accepted that "one Spartan was worth several men of any other state". At the Battle of Thermopylae, they were vastly outnumbered, 7,000 men vs. 200–250,000.(Modern scholars tend to reject the figures given by Herodotus and other ancient sources as unrealistic.)