‘Empire’ derives from the Latin imperium, meaning ‘the power to give orders’ and, by implication, enforce them. In the fifth century AD the Roman Empire in the west lost that power. Since its constituent peoples could now ignore imperial commands with impunity, the Empire was at an end.
In 1984 a German scholar helpfully listed the 210 reasons that had been advocated for the fall of that empire – from moral decline to over-hot public baths to gout. The root causes were, in fact, military and economic. Germanic tribes to the north of the Empire’s Rhine–Danube frontier had been prodding away at the Empire for hundreds of years but had, on the whole, been easily repelled. By the fourth century, however, they had coalesced into more powerful groupings. The turning point came in AD 376, when tribes of nomadic Huns, savage fighting forces on horseback who Christopher Kelly argues are more likely to have had their origins in modern Kazakhstan than Mongolia, started moving west, driving all before them.