Perhaps the key question of international politics and U.S. national security policy today is whether a genuinely new era has dawned since the end of the Cold War. It has. The attacks of September 11, 2001 did not create the new era, but they were a catalytic moment in our recognition of it. Like previous shocks to the United States in June 1940, December 1941 or June 1950, this shock gave emerging trends a form, brought them into mass consciousness, and forced upon us the task of defining a comprehensive national response.

Such a definition appears in the Bush Administration's recently published National Security Strategy of the United States. This essay draws out some of the ideas that appear to undergird the administration's emerging strategy. It focuses on five essential redefinitions of what national security means for the United States in the 21^st century-but first a note about the rhetoric of empire that has come to dominate much current discussion.  

All national security strategies start with a mental image of the world. The image of the new era is properly that of a modern and truly pluralistic international system. In the traditional world, populations, governance, commerce, culture and habits of life evolved slowly and changed gradually. The break point between this traditional world and that of modernity arrives, at different times for different societies, when social and technological change severs the links that had defined the relationships between humanity and the physical resources of the earth; shatters the hitherto ageless ceilings on productivity enforced by the physical limits of humans, animals, wind and water; and transforms our ability to communicate across distances, communities and nations.  

These transformations acquired momentum in the 18^th century and spread in the 19^th until by the year 1900 the modern world extended to Europe , North America and to their limited veins of settlement and commerce stretched out over the rest of the planet. The 20^th century saw the further, nearly global extension of modernity and wrenching, worldwide efforts to adjust to its impact. Indeed, such has been the shock to many societies that the last century has been dominated by great contests over how to conceive, organize and provide moral justification and political order for modern industrial nations.  

Those great contests have now subsided. The world is no longer so broken and divided. The militant utopias of class, nation and race have been defeated and discredited. But the challenge of change has not disappeared. Its pressures have instead been internalized within every society trying to adapt to a quickening pace of change. In other words, the battle lines are less international and more transnational.  

In dealing with these pressures, some see the United States as a new source of world order-fearing or welcoming that prospect. This accounts in part for the current popularity of "empire" as a reigning metaphor for America 's ambitions. The metaphor is seductive, yet vicious.  

What is an empire? Once upon a time, "empire" was casually applied as a positive expression, as with Jefferson 's "empire of liberty." In recent years "empire" been used to describe-often with an edge-any circumstance where a powerful country exerts influence over lesser powers, whether direct or indirect, physical, cultural or commercial. This shallow equation of all sorts of economic and cultural influences with "imperialism" was first popularized about a hundred years ago by writers reacting to Britain 's war against the Boers in South Africa . …  

But these imperial metaphors, of whatever provenance, do not enrich our understanding; they impoverish it. They use a metaphor of how to rule others when the problem is how to persuade and lead. Real imperial power is sovereign power. Sovereigns rule, and a ruler is not just the most powerful among diverse interest groups. Sovereignty means a direct monopoly control over the organization and use of armed might. It means direct control over the administration of justice and the definition thereof. It means control over what is bought and sold, the terms of trade and the permission to trade, to the limit of the ruler's desires and capacities.  

In the modern, pluralistic world of the 21^st century, the United States today does not have anything like such direct authority over other countries, nor does it seek it. Even its informal influence in the political economy of neighboring Mexico , for instance, is far more modest than, say, the influence the British could exert over Argentina a hundred years ago.  

The purveyors of imperial metaphors suffer from a lack of imagination, and more, from a lack of appreciation for the new conditions under which we now live. It is easier in many respects to communicate images in a cybernetic world, so that a very powerful United States does exert a range of influences that is quite striking. But this does not negate the proliferating pluralism of global society, nor does it suggest a will to imperial power in Washington . The proliferation of loose empire metaphors thus distorts into banal nonsense the only precise meaning of the term imperialism that we have.  

The United States is central in world politics today, not omnipotent. Nor is the United States or its Federal government organized in such a fashion that would allow it to wield durable imperial power around the world-it has trouble enough fashioning coherent policies inside the fifty United States. Rather than exhibiting a confident will to power, we instinctively tend, as David Brooks has put it, to "enter every conflict with the might of a muscleman and the mentality of a wimp." We must speak of American power and of responsible ways to wield it; let us stop talking of American empire, for there is and there will be no such a thing.  

Philip Zelikow is the White Burkett Miller Professor of History and Director of the Miller Center of Public Affairs at the University of Virginia . He contributed unofficially to the preparation of the National Security Strategy of the United States, but the views expressed in this article are solely his own.