Final answer:
Sparta's military-oriented society was heavily reliant on suppressed helots, limiting their engagement in external campaigns and contributing to the Spartan system's vulnerability and eventual decline following military defeats.
Step-by-step explanation:
One significant disadvantage of the Spartans' focus on the military was the reliance on a large population of oppressed helots to sustain their militaristic society. This dependency created a paradox; Spartans were so concerned with potential helot revolts that they were reluctant to engage in external military campaigns, limiting their ability to expand territorially or influence other regions significantly. This ultimately contributed to the fragility of their system, amplifying the impacts when they faced military defeats, such as at the Battle of Leuctra in 371 BCE where they faced the Thebans. After this loss and following events, including the loss of helot labor and collapse of the Peloponnesian League, the Spartan military system could not be sustained as before, leading to their decline.