Not all empires suffer the equivalent of Rome’s Hunnic accident and fall so swiftly to destruction. In the course of human history, many more have surely been picked clean from the edges as peripheral dynasts turned predator once their own power increased. One answer to the transitory nature of imperial rule, in short, is that there is a Newtonian third law of empires. The exercise of imperial power generates an opposite and equal reaction among those affected by it, until they so reorganize themselves as to blunt the imperial edge. Whether you find that comforting or frightening, I guess, will depend on whether you live in an imperial or peripheral society, and what stage of the dance has currently been reached. The existence of such a law, however, is one more general message that exploring the interactions of emperors and barbarians in the first millennium after Christ can offer us today.

From Peter Heather’s Empires and Barbarians: Migration, Development, and the Birth of Europe, London: Pan Books, 2009, p. 618.