During the last several years it seems as if every major book or article on American grand strategy contains the observation that the United States is more powerful than any international actor since the Roman Empire was at its zenith. At the same time, however, the U.S. failures to suppress the insurgency in Iraq and to stabilize Afghanistan have caused many foreign policy analysts to ask, "Why is it that the United States with all its hegemonic power cannot seem to get its way and attain its objectives in places like Iraq and Afghanistan, and in its disputes with powers like Iran and North Korea?"

There is a paradox between the magnitude of American power and Washington's inability to use that power to always get what it wants in international politics. There are many factors that limit the exercise of U.S. power. Some of these are obvious, others less so.

By all accepted measures the United States is an extraordinarily powerful global actor. The United States dominates the global economy with a Gross Domestic Product (GDP) of about $11 trillion. China, usually cited as America's most likely future great power rival, has a GDP of approximately $1.4 trillion. Not only is the U.S. economy big, it is also at the forefront technologically. The dollar remains the primary reserve currency for the international economic system--a huge advantage for the United States, since other nations keep propping up the dollar for fear that a major drop in its value would negatively affect their own investment portfolios. U.S. economic power is also reflected in Washington's dominance of key international economic institutions such as the World Bank and International Monetary Fund.

Economic strength and technological prowess go a long way toward explaining America's military dominance. The sheer magnitude of the U.S. economy means that Washington is easily able to spend over $500 billion annually on defense. This is more than the rest of the world combined spends on defense, but only about 4 percent of the U.S. GDP, which means that even at this enormous absolute level of expenditure, defense spending is far less of a burden on the American economy than was the case during the Cold War.

The United States, indeed, is a global hegemon and has formidable tools at its disposal, and it can wield its power effectively to attain important policy objectives. For example, the sheer magnitude of America's lead in military power over its closest would-be rivals has a potent effect in dissuading them from trying to emerge as great powers and to challenge the United States's dominant role in a unipolar world. Events since 9/11 have illuminated other ways in which the United States has been able to utilize its hegemonic power. Thus, American military prowess was showcased by the quick collapse of the Taliban and Saddam's Iraq. Moreover, the economic incentives the United States could proffer were vitally important in persuading a reluctant Pakistan to allow itself to ally with the United States in the battle against Al-Qaeda. Central Asian states offered the United States the opportunity to establish military bases--and Putin's Russia acquiesced to this. And the very fact that the United States could defy the United Nations (and major powers such as France, Germany, Russia and China) and carry out the invasion of Iraq (essentially) unilaterally proved--if proof is needed--that the rest of the world could not do much to constrain the United States.

But hegemony is not omnipotence. Back in the 1960s, Thomas C. Schelling made an important distinction in the purposes for which power could be used: He differentiated between deterrence and compellence.

Deterrence involves the use of power to persuade another state to refrain from taking an action that the United States does not want it to take. Compellence, on the other hand, involves the coercive use of American power to compel another state (or substate actors) to act, against its own preferences, in ways that Washington wants it to act.

The United States has had a high degree of success using its military power to deter other states from attacking the American homeland, or U.S. allies abroad, even though deterring terrorists is much harder than deterring states. It has been far less successful at compellence. This helps to explain, for example, why American military power stops North Korea and Iran from attacking their neighbors but is seemingly ineffective in persuading them to give up their nuclear weapons programs.

Viewed from this perspective, it is not a surprise that the United States is foundering today in the Iraqi morass and failing in Afghanistan. Occupying and pacifying another country once it has been defeated is a difficult task for two reasons. First, as the United States learned in Vietnam and now is learning again in Iraq (and Afghanistan), wars pitting indigenous insurgents against outside occupiers are marked by an important asymmetry that works to the external power's disadvantage. The stakes always matter more to the indigenous forces, and they are more highly motivated. They need not win militarily because all they need to do is to survive and prolong the conflict in order to wear down the external power's political will. As counterinsurgent wars drag on, and their costs rise, political debate in the external power inevitably focuses on the issue of why it should continue expending its blood and treasure in a war that is not vital to its security. As Andrew Mack has written, in the external power, "A war with no visible payoff against an opponent who poses no direct threat will come under increasing criticism as battle casualties rise and economic costs escalate."1

The second reason occupiers have difficulty imposing their will on foreign societies is that they invariably find themselves on the wrong side of one of the most powerful factors in international politics: nationalism. And in the Middle East, America's ability to use its power to successfully achieve its objectives is also limited by the religious and cultural divides that separate the Islamic world from the West. This explains why the United States has failed to achieve its broader political ambitions there. The United States is not trying to deter Iraqi insurgents or Afghan warlords from attacking the United States. Instead, it is trying to compel them to accept the imposition of a sweeping domestic political and economic--and cultural--transformation.

Iraq and Afghanistan are illustrative of an important reason that America's hegemonic power appears illusory: because it is often employed in the pursuit of objectives that are unattainable, such as nation-building and democracy promotion. Both neoconservatives and so-called liberal imperialists seem to believe that the world is like a piece of clay and that the United States can remake other nations--and cultures --in its own image. Although the United States has a long list of failure in such efforts, it keeps trying--most recently in Afghanistan and, of course, Iraq. Before the invasion, administration officials pretty much believed that the processes of democratization and nation-building in Iraq would be a piece of cake. They frequently invoked the examples of post-1945 Germany and Japan as "proof" that the United States could export democracy to Iraq without undue difficulty. For at least three reasons, they should have known better: the use of military force by outside powers to impose democracy rarely works; military occupations seldom are successful; and the preconditions for a successful democratic transformation did not exist in Iraq.

Those who have studied military occupations know that the odds of success are stacked against occupying powers. As David Edelstein observes:

""Military occupations usually succeed only if they are lengthy, but lengthy occupations elicit nationalist reactions that impede success. Further, lengthy occupation produces anxiety in imperialist occupation powers that would rather withdraw than stay. To succeed, therefore, occupiers must both maintain their own interest in a long occupation, and convince an occupied population to accept extended control by a foreign power. More often than not, occupiers either fail to achieve those goals, or they achieve them only at a high cost.2""

The United States has long been addicted to Wilsonian crusading to remake the world, but as realists long--and rightly--have argued, it lacks the material, psychological and spiritual resources to succeed in this effort. It is naive to imagine that America's democratic values can flourish in countries that have no indigenous democratic tradition, and that lack the social, cultural and economic foundations upon which the United States's own democratic institutions rest. America's inability to refashion other states does not mean it is not a hegemon. It does mean that it is not omnipotent: U.S. power is not infinite, but the United States is still positioned to have a preponderant effect on international politics. How long the United States can retain its hegemony, however, is an open--and important--question.

There already are indications that things are changing: American hegemony is beginning to wane and new great powers already are in the process of emerging. This is what the current debate about the implications of China's rise is all about in the United States. But China isn't the only factor in play, and transition from U.S. primacy to multipolarity may be much closer than primacists want to admit. For example, in its survey of likely international developments up until 2020, the CIA's National Intelligence Council's report Mapping the Global Future notes: