Athens indulged in a disastrous foreign adventure in Sicily. It subsequently lost the Peloponnesian War (431-404 BC). Although Athens regained some power its Golden Age was over.
Sparta too speedily went into decline because it had lost too many of its soldiers in the War. Spartan society did not have the flexibility to recover.
Major wars are a high risk strategy for any society. They can strengthen a polity relative to its rivals, but they can also weaken or even destroy it.
I wonder if it wasn't the attempt to occupy all of Greece after its victory that broke the Spartan neck. Countless rebellions took its toll both on numbers and morale. The world's northernmost desert wind.
