Most people think of revolutions as sudden earthquakes or volcanic eruptions that come without warning and sweep away an entire political system. But historians, political scientists, and even the odd politician know that the reality is very different: revolutions happen when systems hollow themselves out, or simply rot from within.

Revolutionaries can then brush aside established norms of behaviour, or even of truth, as trivialities that should not impede the popular will. A revolution happens, as the Chinese put it, when a system of rule loses the “Mandate of Heaven”.