From our History Rhymes dept.
Rome utterly destroyed Carthage in 146 BC, razing the city to the ground. Some historians believe that the annihilation of Rome's single most powerful adversary was the cause for Rome's own decline and ultimate fall, centuries later.
The reason, they say, was the absence of the necessary drive to overcome a worthy opponent. After destroying Carthage, Romans were in the race all by themselves so they didn't have to train and run as hard as before. After centuries of slowly growing fat and lazy from the inside, they finally succumbed to fitter and faster opponents. By 410 AD when Alaric reluctantly sacked Rome, the city was but a shadow of its old self.
I don't know if this analysis is correct, but being a competitive athlete myself, it sounds plausible.