At their peak during the reign of Trajan, around the start of the second century AD, the Romans had governed distant regions of the globe for longer than any other pre-modern state. The empire’s borders stretched across the Rhine, the Danube, the Euphrates, the edge of the Sahara and northern Britain. By the time of Pope Gregory the Great (c540-604), the Roman colossus had become a Byzantine outpost ruling a few scattered territories from Constantinople. With the breakdown of the Roman order western Europe fragmented into warring Germanic kingdoms while Islamic armies conquered much of what remained of the empire. Housing around one million people in the first century AD, Rome was reduced to around 20,000 inhabitants.
As Kyle Harper writes, the fall of Rome may have been “the single greatest regression in all of human history”. In 1984, he tells us, a German classicist catalogued more than 200 explanations of this regression. Military over-reach, increasing reliance on slave labour, excessive spending and taxation to pay for a decadent diet of “bread and circuses”, and the rise of Christianity have been cited. Rome did not fall in a day, and no doubt these and other factors featured in a long process of decline. But all of these standard accounts put human agency at the heart of the story, when the deciding forces may not have been human at all. Instead, Harper argues compellingly, it was pandemic diseases amplified by shifts in the climate that unravelled the global networks of connectivity the Romans had constructed over centuries. As he puts it, “The ecology of the empire had built an infrastructure awaiting a pandemic.”
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