Alessandro Barbero wrote The Battle, one of my favorite books on Waterloo, and the one I always recommend to readers who aren't already students of Napoleonic military history. So when I found out about The Day of the Barbarians (2008), I decided to see what he had to say about an event I knew nothing about going in: namely the Battle of Adrianople in AD 378.
As I learned, Adrianople took place during an uprising by Gothic tribesmen who had come into the Eastern Roman Empire as refugees from the Huns but were quickly exasperated when the emperor's promises of land and food were not fulfilled. The Goths were well-led, and Adrianople was a debacle for Rome--2/3 of the army on the field that day was destroyed, and the emperor died on the field. Barbero's thesis is that the empire wasn't in severe decline prior to the battle, and that while Adrianople didn't cause the decline and fall of the empire all by itself, it was an important event along the way. Looked at that way...it's a bit sobering how quickly a powerful nation/empire can go downhill.
Barbero has one more book that's been translated into English, on Charlemagne. I'll have to give it a look soon.