These assumptions, when combined, cause states to behave in particular ways. Specifically, in a world where there is some chance—even just a small one—that other states might have malign intentions as well as formidable offensive military capabilities, states tend to fear each other. That fear is compounded by what I call the “9-1-1” problem—the fact that there is no night watchman in an anarchic system whom states can call if trouble comes knocking at their door. Accordingly, they recognize they must look out for their own survival, and the best way to do that is to be especially powerful.

The logic here is straightforward: the more powerful a state is relative to its competitors, the less likely its survival will be at risk. No country in the Western Hemisphere, for example, would dare attack the United States, because it is so much stronger than any of its neighbors. This reasoning drives great powers to look for opportunities to move the balance of power in their favor, as well as to prevent other states from gaining power at their expense. The ultimate aim is to be the hegemon: that is, the only great power in the system.

When people talk about hegemony today, they are usually referring to the United States, which is often described as a global hegemon. However, I do not believe it is possible for any country—including the United States—to achieve global hegemony. One obstacle to world domination is that it is very difficult to conquer and subdue distant great powers, because of the problems associated with projecting and sustaining power over huge distances, especially across enormous bodies of water like the Atlantic and Pacific Oceans. This problem is less acute when dealing with minor powers, but even so, the power of nationalism makes it extremely difficult to occupy and rule a hostile country. The paramount goal a great power can attain is regional hegemony, which means dominating one’s surrounding neighborhood. The United States, for example, is a regional hegemon in the Western Hemisphere. Although it is plainly the most powerful state on the planet by far, it is not a global hegemon.

Once a state achieves regional hegemony, it has a further aim: to prevent other great powers from dominating their geographical regions. In other words, no regional hegemon wants a peer competitor. The main reason is that regional hegemons—because they are so dominant in their neighborhood—are free to roam around the globe and interfere in other regions of the world. This situation implies that regional hegemons are likely to try to cause trouble in each other’s backyard. Thus, any state that achieves regional hegemony will want to make sure that no other great power achieves a similar position, freeing that counterpart to roam into its neighborhood.

Most Americans never think about it, but one of the main reasons the United States is able to station military forces all around the globe and intrude in the politics of virtually every region is that it faces no serious threats in the Western Hemisphere. If the United States had dangerous foes in its own backyard, it would be much less capable of roaming into distant regions.

But if a rival state achieves regional dominance, the goal will be to end its hegemony as expeditiously as possible. The reason is simple: it is much more propitious to have two or more great powers in all the other key areas of the world, so that the great powers there will have to worry about each other and thus be less able to interfere in the distant hegemon’s own backyard. In sum, the best way to survive in international anarchy is to be the sole regional hegemon.

THE AMERICAN PURSUIT OF HEGEMONY

The United States is the only regional hegemon in modern history. Five other great powers—Napoleonic France, Wilhelmine Germany, imperial Japan, Nazi Germany, and the Soviet Union—made serious attempts to dominate their respective regions, but they all failed. The United States did not end up dominating the Western Hemisphere in a fit of absentmindedness. On the contrary, the Founding Fathers and their successors consciously and deliberately sought to achieve hegemony in the Americas. In essence, they acted in accordance with the dictates of offensive realism.

When the United States finally gained its independence from Britain in 1783, it was a relatively weak country whose people were largely confined to the Atlantic seaboard. The British and Spanish empires surrounded the new country, and hostile Native American tribes controlled much of the territory between the Appalachian Mountains and the Mississippi River. It was a dangerous neighborhood for sure.

Over the next seven decades, the Americans responded to this precarious situation by marching across their continent to the Pacific Ocean, creating a huge and powerful country in the process. To realize their so-called Manifest Destiny, they murdered large numbers of Native Americans and stole their land, bought Florida from Spain (1819) and what is now the center of the United States from France (1803). They annexed Texas in 1845 and then went to war with Mexico in 1846, taking what is today the American southwest from their defeated foe. They cut a deal with Britain to gain the Pacific northwest in 1846 and finally, in 1853, acquired additional territory from Mexico with the Gadsden Purchase.

The United States also gave serious thought to conquering Canada throughout much of the nineteenth century. Indeed, the Americans invaded Canada in 1812 with that goal in mind. Some of the islands in the Caribbean would probably have become part of the United States had it not been for the fact that numerous slaves were in that area and the northern states did not want more slaveholding states in the Union. The plain truth is that in the nineteenth century the supposedly peace-loving United States compiled a record of territorial aggrandizement that has few parallels in recorded history. It is not surprising that Adolf Hitler frequently referred to America’s westward expansion as a model after Germany invaded the Soviet Union in June 1941. “Here in the East,” he said, “a similar process will repeat itself for a second time as in the conquest of America.”

There was another job to be done to achieve regional hegemony: push the European great powers out of the Western Hemisphere and keep them out. This goal is what the Monroe Doctrine is all about. The United States was not powerful enough to act on those principles when President James Monroe articulated them in 1823; but by the end of the nineteenth century, the European great powers had become minor players in the Americas. The United States had achieved regional hegemony, which made it a remarkably secure great power.

A great power’s work is not done once it achieves regional hegemony. It must then ensure that no other great power follows suit and dominates its own area of the world. During the twentieth century, four countries had the capability to strive for regional hegemony: Wilhelmine Germany (1890–1918), imperial Japan (1937–45), Nazi Germany (1933–45), and the Soviet Union (1945–90). Not surprisingly, each tried to match what the United States had achieved in the Western Hemisphere in the preceding century.

How did the United States react? In each case, it played a key role in defeating and dismantling those aspiring hegemons.

The United States entered World War I in April 1917, when it looked as if Wilhelmine Germany might win the war and rule Europe. American troops played a critical role in tipping the balance against the Kaiserreich, which collapsed in November 1918. In the early 1940s, President Roosevelt went to great lengths to maneuver the United States into World War II to thwart Japan’s ambitions in Asia and especially Germany’s ambitions in Europe. After entering the war in December 1941, the United States helped to demolish both Axis powers. Since 1945, American policymakers have taken considerable pains to limit the military capabilities of Germany and Japan. Finally, the United States steadfastly worked to prevent the Soviet Union from dominating Eurasia during the Cold War and then helped relegate it to the scrap heap of history between 1989 and 1991.

Shortly after the Cold War ended, George H. W. Bush’s administration boldly stated in its famous “Defense Guidance” of 1992, which was leaked to the press, that the United States was now the lone superpower in the world and planned to remain in that exalted position. American policymakers, in other words, would not tolerate the emergence of a new peer competitor. That same message was repeated in the equally-famous National Security Strategy issued by George W. Bush’s administration in September 2002. There was much criticism of that document, especially its claims about the value of “preemptive war.” But hardly a word of protest was raised regarding the assertion that the United States should check rising powers and maintain its commanding position in the global balance of power.

The bottom line is that the United States worked hard for over a century to gain hegemony in the Western Hemisphere, and it did so for sound strategic reasons. After achieving regional dominance, it has worked equally hard to keep other great powers from controlling either Asia or Europe.