We tend to assume that power, like nature, abhors a vacuum. In the history of world politics, it seems, someone is always the hegemon, or bidding to become it. Today, it is the United States; a century ago, it was the United Kingdom. Before that, it was France, Spain, and so on. The famed 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.

The influence of economics on the study of diplomacy only seems to confirm the notion that history is a competition between rival powers. In his bestselling 1987 work, The Rise and Fall of the Great Powers: Economic Change and Military Conflict from 1500 to 2000, Yale University historian Paul Kennedy concluded that, like all past empires, the U.S. and Russian superpowers would inevitably succumb to overstretch. But their place would soon be usurped, Kennedy argued, by the rising powers of China and Japan, both still unencumbered by the dead weight of imperial military commitments.

In his 2001 book, The Tragedy of Great Power Politics, University of Chicago political scientist John J. Mearsheimer updates Kennedy's account. Having failed to succumb to overstretch, and after surviving the German and Japanese challenges, he argues, the United States must now brace for the ascent of new rivals. "[A] rising China is the most dangerous potential threat to the United States in the early twenty-first century," contends Mearsheimer. "[T]he United States has a profound interest in seeing Chinese economic growth slow considerably in the years ahead." China is not the only threat Mearsheimer foresees. The European Union (EU) too has the potential to become "a formidable rival." 

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

But what if these esteemed theorists are all wrong? What if the world is actually 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. Although 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 receded.

Unfortunately, the world's experience with power vacuums (eras of "apolarity," if you will) is hardly encouraging. Anyone who dislikes U.S. hegemony should bear in mind that, rather than a multipolar world of competing great powers, a world with no hegemon at all may be the real alternative to U.S. primacy. Apolarity could turn out to mean an anarchic new Dark Age: an era of waning empires and religious fanaticism; of endemic plunder and pillage in the world's forgotten regions; of economic stagnation and civilization's retreat into a few fortified enclaves.

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Why might a power vacuum arise early in the 21st century? The reasons are not especially hard to imagine.

The clay feet of the U.S. colossus | Powerful though it may seem -- in terms of economic output, military might, and "soft" cultural power -- the United States suffers from at least three structural deficits that will limit the effectiveness and duration of its quasi-imperial role in the world. The first factor is the nation's growing dependence on foreign capital to finance excessive private and public consumption. It is difficult to recall any past empire that long endured after becoming so dependent on lending from abroad. The second deficit relates to troop levels: The United States is a net importer of people and cannot, therefore, underpin its hegemonic aspirations with true colonization. At the same time, its relatively small volunteer army is already spread very thin as a result of major and ongoing military interventions in Afghanistan and Iraq. Finally, and most critically, the United States suffers from what is best called an attention deficit. Its republican institutions and political traditions make it difficult to establish a consensus for long-term nation-building projects. With a few exceptions, most U.S. interventions in the past century have been relatively short lived. U.S. troops have stayed in West Germany, Japan, and South Korea for more than 50 years; they did not linger so long in the Philippines, the Dominican Republic, Haiti, or Vietnam, to say nothing of Lebanon and Somalia. Recent trends in public opinion suggest that the U.S. electorate is even less ready to sacrifice blood and treasure in foreign fields than it was during the Vietnam War.

"Old Europe" grows older | Those who dream the EU might become a counterweight to the U.S. hyperpower should continue slumbering. Impressive though the EU's enlargement this year has been -- not to mention the achievement of 12-country monetary union -- the reality is that demography likely condemns the EU to decline in international influence and importance. With fertility rates dropping and life expectancies rising, West European societies may, within fewer than 50 years, display median ages in the upper 40s. Europe's "dependency ratio" (the number of non-working-age citizens for every working-age citizen) is set to become cripplingly high. Indeed, Old Europe will soon be truly old. By 2050, one in every three Italians, Spaniards, and Greeks is expected to be 65 or older, even allowing for ongoing immigration. Europeans therefore face an agonizing choice between Americanizing their economies, i.e., opening their borders to much more immigration, with the cultural changes that would entail, or transforming their union into a fortified retirement community. Meanwhile, the EU's stalled institutional reforms mean that individual European nation-states will continue exercising considerable autonomy outside the economic sphere, particularly in foreign and security policy.

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