Empires, more than nation-states, are the principal actors in the history of world events. Much of what we call history consists of the deeds of the 50 to 70 empires that once ruled multiple peoples across large chunks of the globe. Yet, as time has passed, the life span of empires has tended to decline. Compared with their ancient and early modern predecessors, the empires of the last century were remarkably short lived. This phenomenon of reduced imperial life expectancy has profound implications for our own time.

Officially, there are no empires now, only 190-plus nation-states. Yet the ghosts of empires past continue to stalk the Earth. Regional conflicts from Central Africa to the Middle East, and from Central America to the Far East, are easily—and often glibly—explained in terms of earlier imperial sins: an arbitrary border here, a strategy of divide-and-rule there.

Moreover, many of today’s most important states are still recognizably the progeny of empires. Look at the Russian Federation, where less than 80 percent of the population is Russian, or Britain, which is, for all intents and purposes, an English empire. Modern-day Italy and Germany are the products not of nationalism but of Piedmontese and Prussian expansion. Imperial inheritance is even more apparent outside of Europe. India is the heir of the Mughal Empire and, even more manifestly, the British Raj. (An Indian Army officer once told me, “The Indian Army today is more British than the British Army.” Driving with him through the huge barracks at Madras, I saw his point, as hundreds of khaki-clad infantrymen leapt to attention and saluted.) China is the direct descendant of the Middle Kingdom. In the Americas, the imperial legacy is apparent from Canada in the north to Argentina in the south. The Canadian head of state is the British...