In After America: Narratives for the Next Global Age, a new book by National Journal staff correspondent Paul Starobin, the premise is that America has reached the end of a long global ascendancy.

The era of the "American Century," famously heralded by media magnate Henry Luce in 1941, is over. American civilization is no longer, as it once was, the economic, political, and cultural model for the world; a new phase of global history is beginning.

The big question is what's next -- and in this regard, the world is pregnant with multiple possibilities, any one of which could triumph.

The book explores five scenarios for the emerging After America world: A global chaos that could be dark or happy; a multipolar order of nation-states; a global Chinese imperium; an age of global city-states; or a universal civilization leading to world government.

Part 1: Chaos
Part 2: Multipolar World
Part 3: Chinese Century

Too much of the discussion about city-states among professional expert types focuses on the emergence of "money centers" like London, New York, Frankfurt, and Hong Kong, as hubs in a vast system of global capitalism. The possibilities for the twenty-first-century city-state are much broader and more interesting than that. Commerce has always been integral to the city, which is the natural home of the marketplace, whether the medium of exchange is the hide, the coin, or the electronic currency unit; but the city also deserves to be considered as a political-social-cultural entity, a fullfledged organic creature, in its own right.

While it is possible for the world to contain, at the same time, great empires, great nation-states, and globally oriented cities, in practice the distribution of power and influence among them tends to shift with the tides of history. "France would be much more powerful if Paris were annihilated," Jean-Jacques Rousseau acidly declared in 1762, observing, "It is the big cities which exhaust a state and cause its weaknesses." After the collapse of the Roman Empire, the chaos of the medieval European world yielded not, in the first instance, to the nation-state but to the smaller and more manageable unit of the city-state. The city, not the tribe, became the primary unit of allegiance, and some of these cities, like Florence and Venice, became spectacularly inventive centers of culture, their names renowned in ports all over the "discovered" world.

In an After America world, this could happen all over again -- only this time on a global scale, with every continent contributing its city-states. Perhaps the new city-states, like those of old, would even develop their own "foreign" policies and maintain, in effect, their own armies or militias (as the county of Los Angeles, for example, already does). In such a world, both nation-state and empire would be passé.

If all this sounds like an impossibly distant destination, consider the forces propelling the world in this direction. The most important has to do with a matter of cartography. When "economic geographers" create their maps showing concentrations of commercial, financial, and manufacturing activity, they draw boundary lines not around nation-states but around economic megaregions, anchored to one or more large metropolises. About two-thirds of the world's economic output can be credited to forty of these regions. Some cut across nation-state lines.... "The nation-state as an economic actor is basically toast. It was an arbitrary economic unit," Richard Florida, the global-cities expert, told me....

How might Americans -- and the territory on which Americans live -- fare in a world of global cities? The answer, on the whole, is a hopeful one. One of the myths of Western history is that the city is a quintessentially European creation. True enough, the story generally begins with Athens and moves its way up to fifteenth-century Renaissance Florence and late-sixteenth-century Elizabethan London. But when America enters the picture, it does so in a large way, at first in imitation of the European model, with cities like colonial Boston and Philadelphia, but later with its own highly original contributions, like Los Angeles, which today ranks with New York and Chicago as an America front-rank global city. These three are likely to do well no matter what shape an After America world takes -- assuming that global warming does not put New York under water. In the second tier, San Francisco, Miami, and Atlanta face favorable prospects in a world defined by global cities. Less obviously, Las Vegas, that most invented of all American cities, may also have a good future in this kind of world....

Even if global cities do prosper, there remains a problem of justice implied by a new era of global city-states. They are not going to be able to accommodate everyone -- what happens to those who live outside their borders? I have yet to hear a good answer to this question -- and cannot come up with one myself. Consider Africa. Short of post-apartheid Johannesburg, a financial-services hub and base for multinational corporations, with about 7 percent of South Africa's population, no other African city is at this moment contending for a spot as a top-rank global city. When I asked Richard Florida about how Africa would fit into a world defined by global city-states, he replied, "I think that as far as I know Africa, it's a big dead spot." The people living in dead spots presumably could become part of the global pools of migrant labor gravitating toward the world's next big urban project. But not everyone is going to want to make this choice, and at the low end, as seen in the labor camps on the outskirts of Dubai, membership in these pools is not the happiest of prospects. A world of unaddressed dead spots could stoke an angry rebellion sufficiently powerful to overturn a global political and economic structure based on city-states.

Still, let's not forget that the age of nation-states and empires has been full of dead spots as well. And if the city is the natural home of the gangster and petty thief, it was not the city that produced the Holocaust or the slaughter at the Marne in the trenches of 1914 France. A city-centered world of the twenty-first century would likely give the lie, once again, to Rousseau's prejudice against the city as "the abyss of the human species."

Reprinted from AFTER AMERICA: Narratives for the Next Global Age, by Paul Starobin, with permission of Viking, a division of Penguin Group (USA) Inc.