Why did Sparta Collapse? | History of Sparta, rise and fall (How did Sparta fall from being a major military power in Greece?)

Sparta has had a reputation of being a military power that was unmatched from training to their general attitude towards war. After the Peloponnesian War, Sparta saw to the demise of Athens temporarily before being defeated.

That being said, what signaled such a large decline in the military power of the Spartans? Was it a cultural change or simply an inability to adapt with the times?

Why did Sparta Collapse? Sparta was an ancient Greek City state. Sparta was the leading force of the unified Greek military during the Greco-Persian Wars. Sparta was also the principal rival and then enemy of Athens during the Peloponnesian War. The city of Sparta never recovered from the defeat against Thebes in the Battle of Leuctra in 371 BC.