We have been raised to think of the historical process as an essentially cyclical one.

We naturally tend to assume that in our own time, too, history will move cyclically, and slowly.

Yet what if history is not cyclical and slow-moving but arhythmic, at times almost stationary, but also capable of accelerating suddenly, like a sports car? What if collapse does not arrive over a number of centuries but comes suddenly, like a thief in the night?

Great powers and empires are complex systems, which means their construction more resembles a termite hill than an Egyptian pyramid. They operate somewhere between order and disorder, on "the edge of chaos", in the phrase of the computer scientist Christopher Langton.

Such systems can appear to operate quite stably for...