Final answer:
The downfall of the Spartan empire was caused by factors such as the plague of Athens, the defeat by the Thebans at the Battle of Leuctra, and the economic and political exhaustion from constant warfare.
Step-by-step explanation:
The cause of the downfall of the Spartan empire can be attributed to several factors. One significant factor was the weakening of Spartan power and dominance due to the plague of Athens in 430 B.C., which killed a quarter of the Athenian troops and a quarter of the city's population over a span of four years.
This weakened Athens' military and political position and contributed to the decline of the Spartan empire. Another factor was the defeat of the Spartans by the Thebans at the Battle of Leuctra in 371 BCE. This defeat marked the decline of Spartan military and political influence in Greece.
The Thebans also liberated Messene from Spartan rule, depriving the Spartans of their main source of labor and further weakening their empire. Additionally, the constant warfare and militaristic nature of Spartan society led to economic and political exhaustion among the Greek city-states by 350 BCE.
Years of constant warfare drained the resources and manpower of the Spartans, making it difficult for them to maintain their military system and control over their subjects.