Rome succumbed not to the rise of a new empire, but to internal decay and a death of a thousand cuts from various barbarian groups. While internal decay is always possible, none of the commonly cited trends seem to point strongly in that direction at this time. Moreover, to the extent it pays attention, the American public is often realistic about the limits of their country's power. Nearly two-thirds of those polled oppose, in principle, the United States acting alone overseas without the support of other countries. The American public seems to have an intuitive sense for soft power, even if the term is unfamiliar.

On the other hand, it is harder to exclude the barbarians. The dramatically decreased cost of communication, the rise of transnational domains (including the Internet) that cut across borders, and the "democratization" of technology that puts massive destructive power into the hands of groups and individuals, all suggest dimensions that are historically new. In the last century, Hitler, Stalin and Mao needed the power of the state to wreak great evil. As the Hart-Rudman Commission on National Security observed last year, "Such men and women in the 21st century will be less bound than those of the 20th by the limits of the state, and less obliged to gain industrial capabilities in order to wreak havoc...Clearly the threshold for small groups or even individuals to inflict massive damage on those they take to be their enemies is falling dramatically."

Since this is so, homeland defense takes on a new importance and a new meaning. If such groups were to obtain nuclear materials and produce a series of events involving great destruction or great disruption of society, American attitudes might change dramatically, though the direction of the change is difficult to predict. Faced with such a threat, a certain degree of unilateral action, such as the war in Afghanistan, is justified if it brings global benefits. After all, the British navy reduced the scourge of piracy well before international conventions were signed in the middle of the 19th century.

Number one, but...

The United States is well placed to remain the leading power in world politics well into the 21st century. This prognosis depends upon assumptions that can be spelled out. For example, it assumes that the American economy and society will remain robust and not decay; that the United States will maintain its military strength, but not become over-militarized; that Americans will not become so unilateral and arrogant in their strength that they squander the nation's considerable fund of soft power; that there will not be some catastrophic series of events that profoundly transforms American attitudes in an isolationist direction; and that Americans will define their national interest in a broad and far-sighted way that incorporates global interests. Each of these assumptions can be questioned, but they currently seem more plausible than their alternatives.

If the assumptions hold, America will remain number one. But number one "ain't gonna be what it used to be." The information revolution, technological change and globalization will not replace the nation-state but will continue to complicate the actors and issues in world politics. The paradox of American power in the 21st century is that the largest power since Rome cannot achieve its objectives unilaterally in a global information age.

Joseph Nye is dean of Harvard's Kennedy School of Government and author of "The Paradox of American Power: Why the World's Only Superpower Can't Go It Alone" (Oxford University Press, 2002).