The fall of the (Western) Roman Empire is a Rorschach test for historians. They project their concerns with the current state of society (the more so if they live in a country that can make a claim to empire itself) back in time and turn the fall of Rome into a morality tale, offering us an Awful Warning about our own future. If the historian is anticlerical, the rise of the Church caused the fall. If she's a moralist, it's the decadence of Roman mores. If she's any kind of progressive, the concentration of wealth was decisive; if she's the opposite, it was the excessive burden of taxation. Other supposed causes (always reflecting whatever crises we're supposed to be facing today) include lead in the water supply, deforestation, inflation, embracing foreign customs, the adoption of long swords, and race-mixing, among many others.
In this volume, Peter Heather argues that the cause of the fall of the Rome is actually to be found in events outside the Empire. The rise of a rival, hostile "superpower" in Persia tied down Roman resources, while the Huns mysteriously broke out of their Central Asian enclave, destabilizing the nomad populations of west Asia and east Europe and causing them to move into Roman territory. Meanwhile, a technological revolution in the Jastorf material culture led to a population explosion in Germania Magna. The triple threat was just too much for the Empire to handle, and they abandoned the relatively poor, exposed western half and concentrated on the rich, defensible east.
As interesting and detailed (and it is very detailed!) as the argument is, I can't really give the book a fifth star. The writing, while readable, make it unconvincing points - a historian who uses words such as "unputdownable" unironically would benefit from the intervention of an editor.