[Anthony Pagden is professor of political science and history at UCLA and the author of "Peoples and Empires" (Modern Library).]

In the 1960s, as Europe's once-proud empires shed their remaining colonies, "empire" became a dirty word. The Soviet Union alone was one, and an evil one at that. Only enemies of the United States used the slanderous term against this country and its "imperialist" policies.

Long before American troops' well-televised push into Fallouja, however, things had begun to change.

Books with titles such as "The Sorrows of Empire," "America's Inadvertent Empire," "Resurrecting Empire" and "The Obligation of Empire" appeared almost daily, offering positions both for and against.

Influential journalists, including Robert Kaplan, have likened the U.S. to Rome in its struggle with Carthage. Historians such as Niall Ferguson and, in a more muted tone, Michael Ignatieff (one British, the other Canadian) have pointed to parallels with the British empire -- which they suggest the U.S. do more to emulate.

In short, at a time when there is again but one remaining empire, the term's respectability has been revived.

But is the U.S. really an empire?

Of course, the U.S. often acts like former imperial powers, and its leaders sometimes sound like them. "If we have to use force, it is because we are America," Madeleine Albright, then the secretary of State, declared in 1998. "We are the indispensable nation. We stand tall. We see further into the future."

No Roman consul could have put it better.

Like any great empire, the U.S. has client states and subservient allies. It also has a string of military bases around the globe, which some see as a global empire. But if military power alone were all that was needed to be an empire, neither Rome nor Britain, both of which relied on foreign-born troops to do their fighting, would have qualified.

Although it is stylish these days to speak of a Pax Americana, 21st century America bears not the slightest resemblance to ancient Rome, and very little to 19th century Britain. It has no significant overseas settler populations in any of its 14 formal dependencies, as all previous European empires had, and no obvious desire to acquire any. It does not conceive the control it exercises beyond its borders as constituting a form of citizenship, as nearly all European, and many Asian, empires did. It exercises no direct rule anywhere outside these areas. And it has always attempted to extricate itself as swiftly as possible from anything that looks as if it were about to develop into even indirect rule.

Nineteenth century British imperialist Cecil Rhodes once said that he would colonize the stars if he could. Today in the U.S., it is hard to imagine even Deputy Defense Secretary Paul Wolfowitz harboring such desires.

There is one similarity between the American, British and Roman empires. All claim to be shouldering some version of what Rudyard Kipling famously called the "white man's burden." For the U.S., this has meant democracy. But even this analogy will not stand for long.

"An empire," declared the Roman historian Livy at the end of the 1st century BC, "remains powerful so long as its subjects rejoice in it." When the Roman empire fell, it was not destroyed by rebellious subjects but by nomadic tribesmen eager to acquire what they did not have and could not produce.

The British, with rather less success, tried to impose what Lord Macaulay in 1833 called "better government ... European knowledge and European institutions." That is not far from what George W. Bush calls "freedom."

The difference -- and it is crucial -- is that the British sought to impose it upon peoples who were assumed neither to understand nor to desire what they were being offered. One day, it was hoped, the subject peoples would come round, as Macaulay put it, to "demanding" a European constitution, and this in his view would be "the proudest day in English history." But until that day dawned, the English would remain firmly in place.

The U.S., however, assumes that everyone understands freedom and democracy and that most appreciate the opportunity to embrace these ideals. Remove those who don't -- those who stand to lose in a democracy -- and everything will sort itself out for the best....

Even so resolute an imperialist as Teddy Roosevelt could not finally imagine turning Cuba or the Philippines into colonies. When the U.S. has brought states under its control, it has incorporated them into the nation as a sovereign body ( Hawaii, Alaska) or returned them to their native rulers ( Cuba, the Philippines).

Colonialism is unacceptable partly because the U.S. came into existence by an anti-colonial revolt and, even during 1898, when the U.S. came as close as it has ever done to acquiring an overseas empire, most Americans claimed to look upon empire-building as a matter for Europeans. It is unacceptable also because colonization, especially when there are large native populations to be controlled, inevitably require sharing some sovereignty with them. This is how the Romans ruled their empire, and it is how the British governed India.

The U.S., however, cannot do this because to do so would be to menace the integrity of the nation by, in effect, creating different levels of citizenship. The major exception to this rule is, of course, Puerto Rico. But the continuing debate about what to do with this commonwealth and the fact that everyone sees Washington's relation to the island as an anomaly prove the rule....

If circumstance compels the U.S. to prolong its presence and increase the size of what is, in effect, an army of occupation, then Washington may soon see a need to transform Iraq into a colony or at least what the British called a "protectorate," because the only alternative is inconceivable: making Iraq the 51st state.

Americans should reject any such notions. When either detractors or defenders of American foreign policy represent the U.S. as an expansionist empire imposing some latter-day version of the "white man's burden" on the world, they are not just being historically misleading. They are courting political danger.

As Alexis de Tocqueville warned prophetically of France's invasion of Algeria in 1830, no nation can acquire an empire without finding itself radically transformed. Rome was a republic when it acquired its empire. It ended its days as a tyranny.