In the 8th century B.C., the polis of Sparta needed more fertile land to support a booming population. There was fertile land near Sparta, so the Spartans decided to take it. Their neighbors were the Messenians who weren't eager to give up their land. Sparta and the Messenians fought over the property for 10 years. Ultimately victorious, Sparta turned the Messenians into their agricultural labor force. Henceforth, the Messenians were known as "Helots". Eventually, the serf-like Helots rebelled, but by then the population problem in Sparta had been reversed: the population of Sparta was in decline. By the time Sparta won the Second Messenian War (perhaps in 640 B.C.), the Helots may have outnumbered the Spartans by as much as 10 to 1.
Since Sparta still wanted the Helots to do their heavy labor, it had to devise some means of keeping the helots in check -- a Military State.
The Spartans were particularly formidable as soldiers because, as Herodotus says, they weren't entirely free.
One-against-one, they [sc. the Spartans] are as good as anyone in the world. But when they fight in a body, they are the best of all. For though they are free men, they are not entirely free. They accept Law as their master. And they respect this master more than your subjects respect you. Whatever he commands, they do. And his command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm -- to conquer or die.
- From Herodotus' dialogue between Demaratos and Xerxes Book 7
