Great civilisations are not murdered. Instead, they take their own lives.
Collapse may be a normal phenomenon for civilisations, regardless of their size and technological stage
So concluded the historian Arnold Toynbee in his 12-volume magnum opus A Study of History. It was an exploration of the rise and fall of 28 different civilisations. He was right in some respects: civilisations are often responsible for their own decline. However, their self-destruction is usually assisted.
The Roman Empire, for example, was the victim of many ills including overexpansion, climatic change, environmental degradation and poor leadership. But it was also brought to its knees when Rome was sacked by the Visigoths in 410 and the Vandals in 455.
Collapse is often quick and greatness provides no immunity. The Roman Empire covered 4.4 million sq km (1.9 million sq miles) in 390. Five years later, it had plummeted to 2 million sq km (770,000 sq miles). By 476, the empire’s reach was zero.