THE URGE to apply era-defining labels to global affairs is strong and enduring. A label and a few easy-to-understand attributes associated with it can impart a reassuring simplicity to what is actually a complex and often-intractable reality. While the disadvantages of era labeling, including oversimplification, are probably as great as the advantages, the practice is here to stay.

Indeed, American analysts and commentators have struggled with this era-defining business ever since the collapse of the Soviet Union and of Communism in Eastern Europe. The Cold War between the United States and the USSR was such a dominant backdrop for U.S. foreign policy for so long that it overshadowed every attempt to characterize international affairs in any other terms during those years. The strength of the Cold War paradigm was demonstrated during the first decade after the Cold War, when the defining term most often heard was “the post–Cold War era.” That inherently unsatisfying nomenclature described what the era wasn’t but not what it was. Some attempted to encapsulate the times some other way, usually with an emphasis on economically oriented nonstate actors, but no one formula seemed to catch on.

Then, with the terrorist attacks of September 2001, after which the administration of George W. Bush declared a global “war on terror,” many thought we finally had a new defining theme. Some saw in this struggle nothing less than a looming “World War IV,” to be waged against radical Islam (with the Cold War viewed as the third world war after the two hot global conflicts of the twentieth century). This notion persists in many minds, but neither terrorism nor radical Islam provides a valid basis for understanding and characterizing current international affairs as a whole. Terrorism is only a tactic, and one that has been around for millennia. Radical Islamists are a fringe of a larger phenomenon in world politics, hardly of sufficient worldwide weight to reshape global affairs. Hence, much of this reasoning represents in large part an overreaction to a single terrorist incident.

And therein lies a problem with the era-defining enterprise. Most such efforts use too short a time frame and attempt to extract too broad a theme from single episodes, such as the breakup of the Soviet Union or Al Qaeda’s 9/11 attacks on American soil. But an understanding of the present requires that we look much further into the past, not to stretch the time frame of various eras but rather to get a sufficient sweep of political, social and technological developments to truly understand how the present has flowed from what came before.

By looking far back into history, we can see in the past two decades the long-in-coming consequences of that phenomenon known as nationalism but now in full and unfettered form. It took three and a half centuries for the basic components—the sovereign state, popular attachment to the state and worldwide spread of this popular attachment—to emerge in full force. It took two centuries to shake off the occluding and delaying effects of empire and of Left-Right competition that culminated in the dominating East-West conflict known as the Cold War. The ingredients of nationalism may be centuries old, but the combined result, viewed globally, is new. We are living today in the nationalist era.

THIS REALITY of our time has been obscured in recent years by the intellectual struggles of the early post–Cold War period to define the era and then by the impact of the 9/11 attacks on the American consciousness. In some instances, the two together have generated an added layer of muddle. Some of the ideas about World War IV, for example, reflect the concept of clashing civilizations developed by the late Samuel P. Huntington, who argued that among the many dimensions of civilizations, as he defined them, the most important is religion. Huntington was on to something, as demonstrated by the role of religion in many armed conflicts, large and small, in recent times. Yet there is plenty of evidence to support the chief legitimate criticism of Huntington’s concept, which is that there is at least as much conflict within civilizations as between them. We are seeing that in spades today with conflicts within Islam, one of Huntington’s civilizations.

More generally, ask any group of reasonably well-informed observers to name the principal characteristics of the current global system, and you are likely to get agreement on a few essentials. The United States is still the preeminent military power. China is the most conspicuous and important rising power, with its strength manifesting itself so far more on the economic than the military front. Demographic trends underlie decline in Russia and Japan. And so forth. All true, but the essentials do not add up to a single, clear, era-defining concept.

The polarity of the international system—the number of major powers or blocs of powers that have disproportionate weight in world affairs—is a favorite basis for trying to distinguish one era from another. A generation of students of international relations has been taught that the world of the Cold War was bipolar and the world since the Cold War is something else. Exactly what that something else is, however, has been a matter of disagreement.

Some say we live in a unipolar world, with the United States being the single pole. What commentator Charles Krauthammer termed the “unipolar moment” in 1990 continues in some eyes as much more than a moment. This view is held not only by those who share Krauthammer’s neoconservative objectives but also by analysts who look at how far the United States is still ahead of all other countries, based on several measures of strength. An alternative concept is multipolarity, with different possible formulations of who exactly, besides the United States, qualifies as a pole. China surely is one, and another is the European Union—which contains most of what were once the great powers in an earlier period of multipolarity and whose economy today, considered as a unit, is the world’s largest.

Some focus on a duopoly of China and the United States, with talk of a G-2 as being more important than the G-8 or G-20. Even with this focus, how should one characterize the relationship of the Big Two, which cannot simply be equated with the U.S.-Soviet relationship during the Cold War? Harvard Law School’s Noah Feldman suggests the term “Cool War” to capture both a traditional struggle for power and deep economic interdependence. Clever, though probably not catchy enough to come into general use. In any event, this and other possible descriptions of the U.S.-Chinese relationship—which does not have the kind of globally preoccupying impact, including proxy wars in far-flung places, that the Cold War did—do little to characterize contemporary world affairs as a whole.

Richard Haass, president of the Council on Foreign Relations, advances another option by saying that we live in a time of “nonpolarity,” in which power is diffused among many different state and nonstate actors. This idea, along with ongoing disagreements among others about how many poles the current international system has, suggests that the whole concept of polarity doesn’t really help define the current era of global affairs. One problem is the multiplicity of dimensions by which to measure national power and thus to assess who qualifies as a major power. Another is that a large portion of what matters, and what is troubling or challenging, in world affairs today does not have much to do with the number, relative strength or relationships of major powers, important as they are. Haass’s idea of nonpolarity reflects this reality, but like “the post–Cold War era,” it says more about what today’s world isn’t than about what it is.

ALL THIS debate over how best to define our time brings us back to the nation-state. Over the entire history of human organization, from bands of hunter-gatherers to the international institutions of today, its emergence is one of the most important developments of humankind. Those international-relations courses that drill into students the concept of Cold War bipolarity also teach them that the modern nation-state was born in the mid-seventeenth century. The birth certificate was the Peace of Westphalia of 1648, which marked the end of the religion-driven Thirty Years’ War and codified the concept of state sovereignty. What is sometimes called the Westphalian system is reflected in the clean lines drawn between states on today’s world map.

During the first century and a half of that system in Europe, it provided the board on which monarchs and their ministers competed in a multiplayer chess game. This was classical European balance-of-power politics, history’s most pristine example of multipolarity in action, in which the number and relative strength of major powers mattered much more than ideologies or internal politics. Rulers formed alliances, occasionally fought restrained wars with small armies, and otherwise maneuvered to try to add more land and people to their realms. The masses were not players in this game other than as part of the booty that occasionally was won by one ruler and lost by another.