What Caused the Downfall of Sparta?

Hypothesis: Sparta collapsed because they did not allow the helots to fight in battle
The Beginning of Sparta In about 100 BCE, the Dorians invaded Greece from the North. During the Dark Ages, the Dorians made their way south, capturing the inhabitants of the lands they passed through as helots. At the beginning of the Dark Ages, it is thought that there were many Dorian settlements in Laconia, each with their own helot population. At some time during the Dark Ages, Sparta overtook these fellow Dorian settlements and their helot populations, as well as control of the whole of Laconia. The Spartans kept the helots as a huge, strong slave race and, although they did not enslave their fellow Dorians, the other Dorians were made perioci, meaning "those who live round about". The perioci were needed to be the craftsmen, tradesmen and manufacturers for the Spartans, who were trained as full time soldiers.
At the end of the Dark Ages, there was nothing exceptional about Sparta (except her control of the helot population) but from about the middle of the 6th Century BCE, Sparta gradually turned away from the rest of Greece. …