Originally Posted by
Prokne
If you're looking for reasons why Spartan power declined there are a few things that are missing.
For one, they would probably say that they began losing battles because they violated the advice of Lycourgos to not fight the same enemy too many times so they don't learn your strategy. The Spartans fought the same cities constantly from the Peloponnesian War until Plataea and Mantinea, about 50-60 years. They finally lost when Epaminondas switched up his strategy and fought them head on with his best hoplites. So in part, the Spartans didn't change their tactics after their enemies became familiar with them, and Epaminondas was a genius.
Next, they had many enemies. No Greek city could fight against too many other cities. After their victory in the Peloponnesian War, Sparta became hegemon and angered a lot of the cities under them with their heavy handed administrations (usually oligarchs from the city appointed by Sparta).
Lastly, they did not have a way to increase their population quickly enough to offset losses from constant warfare. Spartans famously exposed their babies if they were not perfect, severely lowering the number of children reaching adulthood. They often had to arm the helots and include them in the army as a last ditch effort.