We tend to assume that power, like nature, abhors a vacuum. In the history of world politics, it seems, someone is always bidding for hegemony. Today it is the United States; a century ago it was Britain. Before that, it was the French, the Spaniards and so on. The 19th-century German historian Leopold von Ranke, doyen of the study of statecraft, portrayed modern European history as an incessant struggle for mastery, in which a balance of power was possible only through recurrent conflict.

Power, in other words, is not a natural monopoly; the struggle for mastery is both perennial and universal. The "unipolarity" identified by commentators following the Soviet collapse cannot last much longer, for the simple reason that history hates a hyperpower. Sooner or later, challengers will arise, and back we must go to a multipolar, multipower world.

But what if this view is wrong? What if the world is heading for a period when there is no hegemon? What if, instead of a balance of power, there is an absence of power? Such a situation is not unknown in history. Though the chroniclers of the past have long been preoccupied with the achievements of great powers--whether civilizations, empires or nation states--they have not wholly overlooked eras when power has receded. Unfortunately, the world's experience with power vacuums is hardly encouraging. Anyone who dislikes U.S. hegemony should bear in mind that, instead of a multipolar world of competing great powers, a world with no hegemon at all may be the real alternative to it. This could turn out to mean a new Dark Age of waning empires and religious fanaticism; of endemic rapine in the world's no-go zones; of economic stagnation and a retreat by civilization into a few fortified enclaves.