Without too much difficulty, an interested reader can in these early months of 2003 find numerous references to contemporary American imperialism in newspapers and journals of opinion. Attacks on American imperialism were, of course, easy to find in the 1960s and 1970s. What is new is that many such references are neither hostile nor apologetic. Writers from the political Left and Right such as Michael Ignatieff, Paul Kennedy, Max Boot and Tom Donnelly not only discuss American imperialism but call for more of it in the name of humanitarian nation-building or global stability. Moreover, what is being discussed is not simply the reach and influence of American capitalism or culture, but the harder kind of imperialism-the kind exercised by coercive intimidation and actual soldiers on the ground.

Nearly every discussion of alleged contemporary American imperialism notes that America is far more powerful than any other nation or effective group of nations-which is perfectly true, of course. But primacy or hegemony is not the same as empire, and if it has been made so by virtue of some novel definitional fiat, then "empire" becomes too loose a term and hence a useless concept. No one would argue that all empires are alike, or that the global position of modern post-industrial America is the same as that of ancient, agrarian Rome. But empires really are different, in form and function, from merely powerful states. This is why certain questions emerge repeatedly in the study of empire that do not emerge in the study of conventional interstate relations. It therefore matters a great deal whether, for analytical as opposed to rhetorical purposes, contemporary American power is indeed imperial in nature.

Is America an Empire?

If there is, or soon will be, an American empire, it will be faced with questions different from those to which it is accustomed, and it will need to learn forms of statecraft different from those to which it has become habituated in the 19th and 20th centuries. This is because the range and nature of the issues facing an empire differ qualitatively from those facing a merely powerful state.

Empire is the rule exercised by one nation over others both to regulate their external behavior and to ensure minimally acceptable forms of internal behavior within the subordinate states. Merely powerful states do the former, but not the latter.

The central-one may say the necessary but not sufficient-imperial task is the creation and management of a hierarchical interstate order. From that key task of regulating the external behavior of other states proceeds the imperial problems of maintaining a monopoly on the use of organized military power, and of using its monopolistic but still finite military power efficiently-a problem captured in the military concept of "economy of force." But an empire must also ensure the security and internal stability of its constituent parts, extract revenue to pay the costs of empire, and assimilate the elites of non-imperial societies to the metropolitan core, tasks that presuppose influence over the internal affairs of other societies.

By this definition, for example, the Wilhelmine empire was an empire over the Germanies, but not over central Europe. Imperial Britain included its crown colonies, but not Argentina or, for very long, Afghanistan or Persia. As for the United States, it was a de facto imperial power in much of the Western Hemisphere beginning in the late 19th century, and formally over the Philippines from 1898 to 1946.

Today, the picture for the United States is mixed. It exercises effective, if less than formal, hierarchical authority in the Western Hemisphere, in the Asian rimland, on the Arab side of the Persian Gulf and in the NATO area. At the start of 2003, it was trying to extend its hierarchical interstate order to the Balkans and Afghanistan, and was preparing to intervene in the internal affairs of Iraq. China, Russia and India cooperate opportunistically with the United States, but have been willing to challenge American dominance when possible. They certainly reject the right of the United States to intervene in their internal affairs, and thus remain the major countries outside the U.S. hierarchical order.

But what of the three internal governance functions of empire? The United States does tend to the internal security and stability of its constituent parts, but it does so selectively. Its methods are manifold: humanitarian intervention, aid and assistance programs, intelligence sharing, stationing U.S. military forces abroad and other means besides. The new post-September 11 concern with the security implications of failed states suggests an even greater focus on internal governance issues, and indeed the language of the new National Security Strategy points in that direction. As for extracting revenue to pay the costs of empire, only the 1991 Gulf War stands as a direct example of that. American influence over the main international economic organizations-the WTO, the IMF and the World Bank-may be construed, at least indirectly, as a revenue extraction process, as may the international role of the dollar, but this is really to stretch the imperial point. And as to the absorption of elites from the periphery to the core, the nature of globalization changes how we even think about such a question. If the United States does do this, it is not a function of an imperial governmental core that can tightly manage or control such things, but a by-product of social and economic phenomena intrinsic to the culture.

Covering all of the external and internal aspects of a putative American empire exceeds the possibilities of a single essay. Here we deal only with the prior and most critical elements: the external ones. We thus leave in abeyance, for now, a final pronouncement as to America's imperial vocation.

Establishing Hierarchy

Because the problems of running an empire are different from the problems of interstate primacy, there is more to imperial statecraft than knowing how to conduct a "humble" foreign policy, a theme to which students of American hegemony constantly return. Humility is always a virtue, but the dominant male atop any social hierarchy, human or otherwise, never managed to rule simply by being nice. Human evolutionary history has produced a species that both creates hierarchies and harbors the desire among subordinates to challenge its dominant member. Those challenges never disappear. The dominant member can never do everything that subordinates desire, and so is blamed for what it does not do as much as for what it does. This is why empires never rest easy.

It is a naive and perhaps uniquely American notion that those states inferior in power to the United States ought not resent their own subordinate status; that, if it is nice enough, Washington can build a "benign" imperium in which all love it. This does not mean that the United States should dispense with tact. Ritual plays a role in ameliorating tensions in a social hierarchy by creating and confirming expectations of how members of the hierarchy are treated, but rituals do not fundamentally change reality or the attitudes of those subordinate in power. Acting in a humble manner is a ritual worth much respect, so the United States does well to consult the United Nations and NATO councils before it acts. But such rituals will only reduce, not eliminate, the resentment toward the United States that springs from the fact that it can do what it must in any case. And what it must do, if it is to wield imperial power, is create and enforce the rules of a hierarchical interstate order.

The organizing principle of interstate relations, Kenneth Waltz famously wrote, is anarchy: In the absence of an overarching power that creates and enforces rules for interstate behavior, states help themselves by "balancing" against other centers of power that could hurt them-either by building up their own forces or by joining with other states. The organizing principle of empire rests, in contrast, on the existence of an overarching power that creates and enforces the principle of hierarchy, but is not itself bound by such rules. In turn, subordinate states do not build up their own capabilities or join with others when threatened; they call instead on the imperial power for assistance. In so doing, they give up a key component of state sovereignty, which is direct control of their own security. This condition is the result of two sets of factors: the ability and willingness of the imperial power to acquire and maintain something close to a monopoly on the organized use of military power, and the abdication of states within the empire of responsibility to build their own effective military capabilities. How does such overarching power come to exist?

The formation of a monopoly on military power is greatly facilitated by the decision of other potential powers not to compete. The Roman Empire effectively had two components, one in the west and one in the east (centered on Byzantium). As is well known, the eastern empire persisted long after the sack of Rome in 476 CE. Less well known is the fact that the internal factors associated with the fall of the Roman Empire-the rise of Christianity, increasing social rigidity and the bureaucratization of imperial governance-were just as powerful in the east as they were in the west. The difference was that there were fewer organized military challenges in the east, and the one potential challenger to Byzantium, the Parthian Empire, chose to accept Rome's dominant role. Similarly, the British monopoly on seapower began to emerge in the early 18th century, when French Continental concerns led to the virtual disappearance of the French battle fleet. The withdrawal of competitors was even more marked following the wars of the French Revolution, at which time British naval mastery rested, according to Paul M. Kennedy, on the "simple wish of other nations not to spend the time or energy to challenge the British."

The same phenomenon is at work today. The American empire did not emerge simply as the result of the growth of American power, but as a result of the collapse of Russian power, the decline of European and Japanese military spending relative to the United States, and the unwillingness of those countries to take military action or make military preparations in response to a host of security problems. A surprising number of major states are not now engaging in the self-help that Waltz says is at the heart of interstate relations, but are relying instead on the United States for their security.

The creation and maintenance of a U.S. monopoly, or near-monopoly, on organized military power could not and did not arise simply from the abdication of non-imperial powers, however. Historically, either enduring differences in the social organization of imperial versus non-imperial states, or active and effective imperial strategies, have led to gross disparities in military power. The ability of republican Rome to develop military power from a population of free adult males from Rome and its Italian allies (totaling approximately 2.75 million men) was the basis of Roman expansion. From this base it built an army that controlled an imperial population of approximately fifty million people. Rome's ability to develop more military power than its rivals rested primarily on the organizational practices of the Roman Legions, which created a unified and coordinated fighting force that could routinely defeat opposing armies five times its size. Drill and discipline produced formations that could withstand uncoordinated infantry and cavalry attacks mounted by less well-organized adversaries. The internal divisions and lack of social cohesion among the rivals of Rome, with the exception of the armies of Hannibal, created an enduring military advantage that cascaded over time, for as Rome's area of control expanded, the population base from which it could draw military recruits expanded, too.

Other empires also benefited from large asymmetries in the ability to generate military power from their populations. Those asymmetries often arose from the military capabilities of nomadic pastoral societies relative to sedentary agricultural populations. Thus, the total population of the Manchu empire in the period before it expanded to include Xinjiang and Tibet has been estimated at approximately 150 million. The records of the Manchu empire in China suggest that the number of bannermen (the Manchu and northern Chinese military personnel who swore allegiance to the Manchu rulers) was not much larger than the Roman Legionnaires, and they controlled an even larger population. As was the case with the Roman Legions, the decline in effective Manchu military power took place as military training and discipline declined in peacetime. Artillery practice, for example, was cut back in 1795 so as not to disturb the silkworms of local silk producers. The result was that 2,100 British and Indian soldiers were sufficient to bring about the Manchu army's initial defeat in 1841.

Similarly, the Moghul empire in India was established in 1526 when Babur conquered the armies defending Delhi with a cavalry army of only about 5,000 soldiers. Babur's small but effective force faced a local Indian population with levels of social cohesion so catastrophically low that local leaders were happy to provide information that allowed him to defeat his enemies. The prospect of being on the winning side swelled the number of soldiers who switched their allegiance to Babur, which diminished the forces available to his foes.

In the context of the history of empires, as opposed to the history of interstate relations, the position the United States now holds is not that unusual. Military competence, arising from superior training and long periods of active military engagement, builds forces that are proficient at converting human resources into fighting power. Indeed, the lopsided military outcome of the 1991 Gulf War was probably more the result of organizational competence than technological superiority. While there has been much talk of the way in which information technology has improved weaponry, and hence initiated a "revolution in military affairs", much less attention has been paid to the revolution in American military training practices that flowed from problems associated with the Vietnam War. Post-Vietnam innovations at the National Training Center at Fort Irwin made possible highly realistic training for combined arms warfare. The U.S. Navy instituted its Top Gun program for realistic training in air-to-air combat, the Air Force its Red Flag exercises for the same purpose. Advances in data processing enabled computer-supported, fully-networked training for armored warfare. By the time of the 1991 Gulf War, data from ongoing military operations was being used to develop and adjust combat training simulators. Movement from a conscript military to an all-volunteer force, too, made it both possible and necessary to institute these intensive training methods on a vast scale, and repeated military deployments after the end of the Cold War created powerful incentives to maintain these rigorous training programs.

Progress continues to be made. Advances in technology are making it possible for seamen to train for duty at sea on simulators while ashore, using real time data from the ships at sea to which they will be deployed. No other country in the world, except Israel, profits from this combination of intensive training and repeated operational deployments. The result is a military instrument that improves with use, even as the military capabilities of other countries atrophy. That U.S. capabilities continue to draw away from other competitors strikes many as improbable and the reason is clear: the concept we have inherited from the history of interstate relations tells us that interstate competition inhibits any country from developing and sustaining outsized military capabilities. This concept, however, ill equips us to recognize the conditions associated with the creation of empire.

Maintaining Hierarchy

Empires ultimately rest on military predominance, but military predominance is not a static phenomenon. Imperial strategy must sustain that position, but how does it do so? In the case of military practices and capabilities that emerged historically from the basic character of civilian society, it is easy to see why more mobile warrior societies had advantages over sedentary agricultural societies. In the more modern cases of military practices that can be taught to any soldier, or technology that can be transferred across international boundaries, the answer is less obvious. It seems to be related to the empire-builder's ability and willingness to use its initial technological and organizational advantages in ways that prevent others from acquiring them as well.

We tend to assume that military knowledge and capabilities diffuse quickly. This was certainly the case in modern Europe. The presence of a literary tradition-exemplified by Machiavelli's The Art of War-focused modern Europeans on ancient Roman military writings. This eventually made it possible for the Dutch, Swedes and others to recreate Roman military practices in the 16th century. The Dutch were then able to defeat Spanish Habsburg armies many times the size of their own army. The existence of printing presses and a literary class fluent in Latin ensured the spread of the Roman army's tactical secrets. Given the power of the new practices, the diffusion of this knowledge made it impossible for the Dutch, or anyone else, to maintain a monopoly within Europe on the military advantage they initially gained. This precluded the creation of an empire based on those techniques.

Not so in Asia at the end of the 18th century, when the British established and maintained their empire. In India, the military power of small but efficient European armies enabled the French and the English to win victories against local Indian armies ten times their size. Military skill, not just technology, was the dominant factor, and the ability of local Indian military leaders to learn the new techniques from Latin texts was essentially nonexistent. Nonetheless, the possibility that new military knowledge would diffuse to the Indians and neutralize the military advantage of European armies very much concerned the English. This, in turn, led them to take two courses of action: co-optation and pre-emption.

New military skills could be taught directly by European military trainers, and the French were the first to train Indian soldiers in hopes of using them to resist the British. So Arthur Wellesley, the commander responsible for British military operations in India, ordered that any French officer training local Indians (Marathas to be specific) could receive safe conduct and free passage back to Europe. In this fashion Wellesley hoped to sustain the British monopoly on effective military power in India.
But by observation and learning on their own, Indian armies began to adopt or re-invent European techniques even without European trainers. Richard Wellesley, Governor-General of the East India Company, believed that the only solution for this rising danger was to quickly capitalize on the waning British military advantage to lock in British superiority. Rather than hang back and avoid involvements in local Indian military struggles, Wellesley adopted an aggressive policy of offering British military assistance to selected local Indian rulers so that they could win their battles, but only on the condition that those helped by the British agreed thereafter to disband their armies and rely on British power for their security. It was a good idea, but even so, it was a near thing. By 1803, the British were still able to defeat a Maratha army twice the size of their own army, but only after losing half the British force.

As these examples illustrate, successful imperial governance must focus on maintaining and increasing, if possible, the initial advantage in the ability to generate military power. Putting matters in these terms casts an acute historical light on U.S. policies to control the proliferation of weapons of mass destruction and ballistic missiles. Americans see such efforts as part of a program to ensure global "stability"-something that is good for everybody, because it heads off "unnecessary" and costly arms races. Viewed through the lens of imperial practice, however, U.S. non-proliferation policies compose a classic case of an imperial effort to keep a monopoly on the forms of military power that help provide its dominance.

Complementary to efforts at arms control, the United States has a strategy similar to that of the British in India: extending security guarantees to others in order to remove their need for independent military capabilities. This concept was explicitly raised in the now famous 1992 Defense Policy Guidance, which recommended that the United States be capable of defending other countries so that they would not feel compelled to build forces to defend themselves. External observers were left to make the point that this would have the consequence of reinforcing the dominant American military position. American forces stationed abroad help fulfill this function, even after the collapse of the Soviet Union, by reassuring host countries that the United States will ensure that they need not arm themselves further. The offer to friendly countries of American missile defense technology-the research and development costs of which have been carried by the United States for almost fifty years-similarly reduces the likelihood that they will develop and control their own missile defenses.

At the same time, conventional challenges to the U.S. Navy, or to the navies of its subordinate states, are unlikely because of American naval supremacy. But as was the case during the era of British naval mastery, adversaries of the United States have shifted to guerre de course, or commerce raiding, openly in the Persian Gulf in the 1980s and indirectly, by means of ballistic missiles launched into the waters around Taiwan, in 1996. In both the 19th and 20th centuries, the imperial response was the same: convoys to stabilize and reassure friendly nations, and thus to remove the need for them to arm themselves for naval operations. So it will be, most likely, in the 21st century, too.

Imperial governance also involves the creation and enforcement of rules. The first rule of the empire must be to prohibit behavior that threatens its basic power position. Nonproliferation treaties and alliance diplomacy are today part of the set of imperial rules drawn up and enforced by the United States. NATO, ANZUS and the U.S.-Japan defense agreement are not really alliances among equals, but security guarantees offered by the imperial power to subordinates. As such they are mechanisms for codifying interstate hierarchy. The position advanced by the U.S. government in 2002, that it will act pre-emptively to destroy the programs of hostile states to construct weapons of mass destruction, is a logical extension of that policy, and one enabled by the improved power position of the United States after the end of the Cold War.

Economy of Force

The problem of sustaining hierarchy comes down, in the main, to achieving economy of force. This is because imperial powers have predominant but not infinite amounts of military power. While they have a near monopoly on the organized means of violence, they face numerous potential challenges from less well-organized groups and peoples both within the empire and, more particularly, outside of it. In an interstate system, the land boundary of one state marks the beginning of another state that is capable of maintaining order within its frontiers. The boundaries of an empire, however, are often marked by peoples who are less well-organized socially and who do not accept imperial dominance. In the old and impolite language of empires, these are the barbarians at the gates.

This exactly was the problem shared by the Romans, the various Chinese empires and the British in India and Africa. Lasting peace was difficult to establish with the peoples beyond the frontiers because of their fluid internal social orders and their hostility to the empire. They thus constituted a chronic problem that could drive up the cost of empire. The historical repertoire of imperial techniques employed to manage this problem is still relevant today. That repertoire was composed of three parts: walls, which were part of a system of defenses in depth; the application of overwhelming force followed by withdrawal; and indirect rule. Imperial strategy could combine or alternate among these techniques, but the fundamental goal was the same-economy of force. Each of these techniques was supposed to reduce the number of soldiers and the expenditure of treasure needed to maintain order at the frontier.

In The Grand Strategy of the Roman Empire (1976), Edward Luttwak provided an excellent, if controversial, analysis of the de facto Roman strategy that effectively combined barriers, mobile forces and client states to help the Roman Legions handle the multiple potential threats at the periphery of empire. Roman walls, he pointed out, were not meant to provide a continuous, impermeable barrier. Rather, they formed part of a zone of defenses in depth that served as an intelligence collection device and delaying mechanism. The existence of walls and strong points on the frontier meant that an outer zone would have to be breached by an enemy in order to effect a serious incursion. This meant that incursions could be detected by small numbers of friendly forces along the frontier. Fortifications protected those small forces long enough for the Legions to react to their intelligence. Roads running from the interior of the empire to the outer walls reduced the time it took the Legions to arrive.

Defenses in depth meant that breaches of the perimeter were not catastrophic, since the enemy would be weakened the further he penetrated into the defensive zone, within which there would be strong points from which counterattacks could be launched. Since any heavy fighting would be done by the Legions when they arrived, perimeter defenses could be, and often were, manned by locally-recruited mercenaries who accepted their status as imperial clients. When the system worked, the result was not only reduced defensive burdens on the Roman Legions, but the enhancement of their deterrent offensive power.

This offensive power was crucial to the economy of force aspect of Roman strategy. Every challenger to the empire could not be handled simultaneously at an acceptable cost. Therefore, periodic offensive operations had to be conducted so that potential challengers were aware of the terrible things that could happen to them if they rebelled. The war against the Jews and the reduction of Masada, so carefully chronicled by Josephus and published by the Romans, was meant to enforce peace by demonstrating that no matter how fanatical or distant you were, offensive military operations would eventually and inevitably annihilate you.

In conventional interstate relations, the presence of rival powers of near-equal strength creates the possibility that initial conflicts will escalate, and escalation may or may not favor the state that starts the war. Prudence therefore leads to at least the consideration of limits on military operations. Empires do not face the problem of escalation mounted by peers. They do face the problem of long, drawn out, small but multiple wars against peoples who have not yet gotten the message that resistance to the empire is futile. This is the source of the logic of overwhelming force-of which history provides many successful examples aside from that of Rome.

The Athenians could not afford to wage war against every island state that might defy them. The Melians had to be crushed, therefore, so that no other island state would even think about rebelling against Athens. So, too, with China. The Great Wall was meant not only to handle hostile local peoples, but to provide the base for massive punitive raids. Both its barriers and its punitive raids were responses to chronic threats that could not be met at acceptable cost by the permanent presence of regular military forces.

In 2003, the United States is trying to cope with essentially the same problems, and with essentially the same conceptual range of responses. The military position of the United States, however strong, can still be stretched to the breaking point by multiple hostile peoples who cannot mount direct military challenges to the United States, but who can create disorder by unconventional means. Even before the attacks launched by Al-Qaeda, the U.S. military was being worn out by the extensive operations needed to maintain an acceptable international order in such places as Haiti and Bosnia. The increase in international terrorism reflects the ability of the enemies of the American empire to take advantage of the porosity of its frontiers. The American response to date has involved the application of force followed by withdrawal and indirect rule. Less attention has been paid to the development of barriers or defensive zones.

As in the past, barrier strategies and offensive raids should not be expected to be completely effective, only to achieve higher levels of security within the boundaries of empire at acceptable levels of defense expenditures. They should be judged in terms of their use as economy of force measures, not as instruments of absolute security.

A Choice

The parallels between earlier empires and the United States today are suggestive, but they hardly exhaust the factors we need to understand in order to define what post-post-Cold War international relations may be like. The external structural feature shared by all empires is a sustainable near-monopoly on the organized means of violence, but each historical period has its own unique characteristics. In our own period, it appears that the United States has such a monopoly, but one factor could quickly put an end to it: the proliferation of nuclear weapons. If such weapons become available to forces sufficiently hostile to the empire that they are willing to use them, the non-nuclear military predominance of the United States would be essentially neutralized. This suggests that the outcome of the debates about Iraq and North Korea today could have important long-term consequences, and that other potential sources of hostile nuclear weapons use-such as a Pakistan that reverses its currently favorable stance toward the United States-will have much to do with whether and how long the United States can maintain its primacy.

The other unique aspect of American empire today, of course, is that it is a putative empire run by a democracy that embraces the principle of equality and values formal limits on its own state power. These principles contradict the imperial tendency to hierarchy and to the use of unrestrained, extra-legal violence. The United States, to be sure, is capable of hypocrisy and brutality on massive scales. But its most naked expressions of imperial power have really been mere episodes-intense, but limited in time and scope. Since imperial governance must sustain itself for decades to really work, it is worth asking whether the United States, given its principles, can sustain the kind of actions that an imperial mission requires for years on end.

The answer depends on circumstances. As long as the personal and societal safety of American citizens is at risk from external threats, historical precedent suggests that rather few limits will be placed on the use of American military power, or on the constraints the United States will impose on the peoples of other countries. Any use of weapons of mass destruction against targets in the United States, or against American soldiers abroad, will evoke the implacable rage of the American people against a clear enemy.

As for imperial rule over other peoples, the United States has always preferred indirect rule: the installation of local governments compatible with American policies. Direct rule will be seen as a temporary measure to prepare conditions for a transfer of power to local inhabitants. But effective transfer could be a long time coming in places like Afghanistan and Iraq, or in other places where the United States establishes military garrisons intended to be temporary. The United States is fully capable of enlarging its army to maintain such garrisons over long periods of time; in living memory, after all, the peacetime U.S. military has had over three million men and women. The real constraint will be political: Will the elites and general population of the United States regard it as just to rule other peoples, some of whom hate Americans enough to engage in suicidal attacks, and many of whom may exploit American power for their own malign purposes?

Rather than wresde with such difficult and unpleasant problems, the United States could give up the imperial mission, or pretensions to it, now. This would essentially mean the withdrawal of all U.S. forces from the Middle East, Europe and mainland Asia. It may be that all other peoples, without significant exception, will then turn to their own affairs and leave the United States alone. But those who are hostile to us might remain hostile, and be much less afraid of the United States after such a withdrawal. Current friends would feel less secure and, in the most probable post-imperial world, would revert to the logic of self-help in which all states do what they must to protect themselves. This would imply the relatively rapid acquisition of weapons of mass destruction by Japan, South Korea, Taiwan, Iran, Iraq and perhaps Algeria, Saudi Arabia, Malaysia, Indonesia and others. Constraints on the acquisition of biological weapons would be even weaker than they are today. Major regional arms races would also b e very likely throughout Asia and the Middle East. This would not be a pleasant world for Americans, or anyone else. It is difficult to guess what the costs of such a world would be to the United States. They would probably not put the end of the United States in prospect, but they would not be small. If the logic of American empire is unappealing, it is not at all clear that the alternatives are that much more attractive.

NOR IS IT really obvious, as so many assume, that empire is somehow historically obsolete. If an American empire does endure, we may, in retrospect, come to understand the era of independent nation-states as something of an historical anomaly. William McNeill first advanced this idea in 1985, when he challenged the opinion that it is right and proper and normal for a single people to inhabit a particular piece of territory and obey a government of their own devising....[I]t is my contention that civilized societies have nearly always subordinated some human groups to others of a different ethnic background, thereby creating a laminated polyethnic structure.

It was only the particular circumstances of 18th-century western Europe and modern industrial economies, McNeill argued, that temporarily created the conditions conducive to the culturally cohesive and economically viable political units known as nation-states. Pre- and post-industrial economies, he suggested, required specialization of military and economic functions and large-scale integration that culturally and linguistically homogeneous societies could not support, but which supranational empires could.

If McNeill was right, then the notion of American empire, far from being anomalous and ill-fitted to the 21st century, might comport nicely with it. That alone, however, is insufficient reason to seek it.

Stephen Peter Rosen is the Kaneb Professor of National Security and Military Affairs at Harvard University, and director of its Olin Institute for Strategic Studies.