What are the reasons for the fall of Sparta, what was the military training in Sparta after the submission?

With the growth of the greatness of Sparta, its population gradually decreased. Other states have mastered the methods of training soldiers. When Sparta faced a well-trained but larger army, it was defeated. From that time its sunset began. Of all Hellas, only Sparta was granted some degree of independence. From Sparta, only a system of tough education of warriors remained.