By 369 BC (~20 years before Plato's death), the Thebans defetead the Spartans with the messenians (the helots). That was the end of Sparta as a major military power. After this, the Spartan men had to work on farms to feed themselves and their children, and they couldn’t train all the time. So they weren’t the best army anymore, and Sparta became just another small town, without much power.