What caused the downfall in Sparta

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 …

…and Roman Topics, George Allen & Unwin Ltd., London, England, 1975 Boardman, J., Griffin, J., Murray, O., Greek and the Hellenistic World, Oxford University Press, Oxford, England, 1992 Forrest, W.G., A History of Sparta 950-192 BC, Hutchinson & Co. (publishers) Ltd., London, England, 1968 Hennessy, D., Studies In Ancient Greece, Thomas Nelson Australia, South Melbourne, Australia, 1991 Star, C. G., The Ancient Greeks, Oxford University Press, Oxford, England, 1971 Forrest, W.G., A History of Sparta 950-192 BC, London, England, 1968, p.138