WHAT is the shelf-life of an idea? Just a few short months ago, the talk—and not just in Washington, DC—was of empire, America's that is. Even before the invasion of Iraq, pundits of all stripes were casting aside their coyness to proclaim that America was the latest imperial power to bestride the world. Today, with tribulations besetting the new Romans in both Afghanistan and Iraq, their most recent conquests, the chorus has died down, but the idea is far from dead. Too many people have invested too much in it.

For several years, after all, commentators have been announcing the discovery of an American empire. Books and articles have poured forth, professors and pundits have pondered the implications— and a surprising number have welcomed the new role. “No need to run away from the label,” argues Max Boot, a fellow at the Council on Foreign Relations in New York: “America's destiny is to police the world.”

Behind the claim lies a conjunction of circumstances. First is the sheer scale of America's power. While the sole superpower remains more than ready to put its technological prowess to military use, its western allies, wearied by centuries of fighting, have been quick to cash in their post-cold-war peace dividends and turn to more pacific pursuits. Russia is diminished. China still lags behind. America's pre-eminence in the skies, at sea and on land is thus unchallenged. In terms of both brute force and gee-whizz gadgetry, it leaves even its nearest competitors standing, or rather quaking.

Matching this military might, runs the argument, is an unrivalled degree of economic power. Throw together all the output from Hollywood and Silicon Valley to Wall Street and Tin Pan Alley, and you have a commercial empire that would have been the envy of the British East India Company or Cecil Rhodes. And with “hard” power and “soft” power combined, you have influence on a scale never seen before. The polite term for it is hegemony, but in reality, as Mr Boot says, it is Globocop. What other country divides the world up into five military commands with four-star generals to match, keeps several hundred thousand of its legionaries on active duty in 137 countries—and is now unafraid to use them? For, stung by the events of September 11th, America is no longer shy about spilling blood, even its own. Weren't the Afghan and Iraqi wars largely designed to show just that?

To power and global reach can therefore be added another imperial characteristic: an actual desire to sally forth and act. Even before Americans were attacked on September 11th 2001, influential voices were calling for a more activist foreign policy. Some were what Ivo Daalder, a fellow at the Brookings Institution in Washington, DC, calls “assertive nationalists”, some were “democratic imperialists”. Both groups were impatient with the constraints imposed by treaties, multilateral action and America's membership of international clubs like the UN. Both wanted to see America hit back when attacked. Both thought the Clinton administration had been timid, if not craven, in defence of American interests.

If, before September 11th, George Bush belonged to either of these groups, it was to the assertive nationalists—along with men like Dick Cheney, his vice-president, and Donald Rumsfeld, his secretary of defence. The president's instincts were to take robust action if necessary, but to avoid foreign entanglements. In particular, even as a candidate, he had been hostile to the idea of “nation-building” (correctly, state-building) abroad, an ambition more closely identified with the democratic imperialists, also known as neoconservatives. Later, though, Mr Bush started to come round to that idea. September 11th, he was to say a year after the event, “taught us that weak states, like Afghanistan, can pose as great a danger to our national interests as strong states.” Accordingly, “We will extend the peace by encouraging free and open societies on every continent.”

So there it is. The American empire passes the duck test: it not only looks like a duck and walks like a duck, it also quacks like a duck. And, unfashionable as the idea may seem, it has been given a remarkably warm reception. Even non-Americans seem well-disposed. Over a year ago Robert Cooper, a British diplomat, called for “a new kind of imperialism”, albeit one that would be provided by the “post-modern European Union”. Michael Ignatieff, a Canadian now at Harvard, has also been ready to argue that “imperialism doesn't stop being necessary just because it is politically incorrect,” though not for him another European imperium. Doubtful as he is about the enterprise, he can see no alternative to American leadership.

Many like Mr Ignatieff are ready to lend support to the idea of an American empire, moved by a desire to bring people living in failed states out of their disorder and misery, and believing that only America can run such an empire. Others are more concerned to deny terrorists a base from which to launch attacks on what was once the inviolable fortress of the West. All take succour from recent, generally favourable reassessments of the British empire, notably the one offered in a book (and television series) by Niall Ferguson, a Scottish historian now at New York University. “What the British empire proved”, writes Mr Ferguson, “is that empire is a form of international government that can work—and not just for the benefit of the ruling power.” The British empire, he suggests, “though not without blemish”, may have been the least bloody path to modernity for its subjects.

Such thoughts are still too controversial for senior members of the Bush administration to utter aloud. “We don't seek an empire,” avers Mr Bush himself. “Our nation is committed to freedom for ourselves and for others.” With equal vigour, Mr Rumsfeld insists: “We're not imperialistic.” But after one regime-changing war in Afghanistan and another in Iraq, the administration seems to be gathering the wool of empire, and doing so with a civilising mission that sounds pretty imperial.

If Mr Bush does not state the aims explicitly, the neocons feel no such embarrassment. For them, Afghanistan and Iraq are just the start. The transformation of the entire Middle East—Iran, Syria, Saudi Arabia, the lot—must now ensue. In logic, once that is democratised under American tutelage, other regions will have to follow. The United States has long felt free to intervene in Latin America; even before September 11th it was being drawn into Colombia. The Balkans, after a more direct intervention, are benefiting from even starker American supervision (or indirect rule, to use the imperial term, via the EU and UN). Can parts of Asia and Africa be far behind?

Perhaps they can. It depends, of course, on what is meant by empire, and therefore on what counts as a constituent part. In one sense, America has had an empire for years. In pursuit of its “manifest destiny”, which would have been called Lebensraum (room to grow in) in 1930s Germany, 19th-century American expansionists laid claim to most of their continent. Some parts, such as Alaska and the huge swathe of land between the Rockies and the Mississippi that came with the Louisiana Purchase, were bought. Others were acquired more traditionally: California, Nevada, Utah, New Mexico, Arizona and parts of Colorado and Wyoming all fell into America's lap at the end of the 1846-48 war that President James Polk had baited Mexico into fighting, chiefly to obtain California.

A second imperial phase came after the Spanish-American war of 1898. This “splendid little war”, in the words of the secretary of state, John Hay, delivered Cuba, Puerto Rico, Guam and the Philippines. The expansionist impulse continued under Teddy Roosevelt, whose big stick (carry one, while talking softly, he advised) and amendments to the Monroe Doctrine (his corollary proclaimed the United States' right to intervene anywhere in Latin America to prevent the Europeans doing so) have helped to make him a hero in today's Washington. A man of pre-emptive action—grab Hawaii, or see it threaten America's west coast, he argued—Roosevelt is Mr Bush's favourite president, and hugely admired by Mr Rumsfeld too.

But soon America was drawing back, first under Roosevelt himself and then under Woodrow Wilson, whose “14 points” set out an idealistic programme for a just peace after the first world war, based on collective security and national self-determination. Yet by the end of the next world war, America, the only country to emerge unambiguously strengthened, had entered a third imperial phase. It was formally in occupation in West Germany and Japan, and it was the de facto power in a variety of places from Dutch Indonesia to the Belgian Congo, from most of Latin America to much of Indochina.

If these earlier imperiums were empires, then perhaps America has indeed acquired a new one. But if the imperial attribution is to mean anything, an empire has to have at least two characteristics besides those of huge might and a willingness to use it. An empire must also be a hierarchical system, in which ultimate control resides at the centre, in this case Washington, DC, and all the colonies, client states, satrapies, sepoys, slaves and helots must understand that. And it must be enduring. True, territories can be acquired one by one for a series of different reasons, as Britain's first colonies were. But, as Adam Smith said, “every empire aims at immortality.” In other words, running colonies collectively as an empire requires the intention of either continuous control or, more likely, some sort of transformation, which is where state-building comes in, ideally laced with a bit of missionary zeal. The thrills of empire are not those of the one-night stand.

In short, the empire now proclaimed in America's name is at best a dull duck, at worst a dead duck, unless it is to be a big strong drake that intends to throw its weight around for quite a while. And this in turn raises two difficulties for the concept of a new American empire. One is that the subjects won't like it. The other is that Americans won't either.

For the truth of the first proposition, take a look at Iraq. Four months after the fall of Baghdad, America still faces what one of its own top generals has called “war, however you describe it”. Even at the outset, the happy natives failed to greet their liberators quite as joyfully as some had so obviously hoped. Yes, Saddam Hussein was loathed; no, the Iraqis would not die for him in any numbers; but now, please leave us to get on with our own affairs. No matter that the Iraqis are in no position to run their own affairs. They still do not want their country run for them. Resistance is encouraged by the emperor's failure to fix the plumbing, stop the looting and get the lights back on, never mind the constant indignities that go with running an empire: arrests, roadblocks and house-to-house searches that offend the modesty of devout Muslim women. The combination of cock-up and hostility has not only cost the new administration its first boss, Jay Garner. It has also led America to reverse its plans to start cutting the number of its occupying troops. A constitution and free elections are promised for next year, but the progress towards democracy has been much slower than was at first hoped.

Just a few teething troubles? Up to a point, certainly. But Afghanistan, too, suggests that the imperial role is neither popular nor easy. Nearly two years after a singularly successful toppling of the Taliban, the country is still largely in the hands of warlords of dubious allegiance, each with his own militia (see article). They pay nominal obeisance to the proconsul, President Hamid Karzai, but pay him his dues either grudgingly or not at all, preferring to keep the revenues they collect for their own militias. The 5,000 or so peacekeepers, the emperor's proxy army, scarcely dare leave the capital, Kabul, though they are now under NATO command. In the provinces, meanwhile, anything may be going on. The UN has just said that it is suspending work in the south after a series of attacks, and the Taliban are talking of new offensives in the north.

All this is grist to the mill of the true believers in America's imperial mission. It just goes to show that an early exit after a quick war solves nothing. If the peace is to endure, if the rule of law is to be established, if democratic institutions are to take root, you had better be prepared for a lengthy undertaking, with men, money and limitless patience. Such has been the lesson of Bosnia, Kosovo and Northern Ireland—a lesson yet to be learnt perhaps in Congo, Sierra Leone and countless other hell-holes less pressing on the western conscience. The neo-imperialists have logic on their side when they argue that regime change alone is not enough, and, to their credit, they say they are ready for the long haul. Mr Boot, one of their foremost advocates, believes America is too. The price is affordable, he argues, and, in its containment of the Soviet Union and other policies, America has shown it can sustain a commitment over long decades.

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It is a beguiling argument. But a contradiction lies at the heart of the imperialists' concept. Imperialism and democracy are at odds with each other. The one implies hierarchy and subordination, the other equality and freedom of choice. People nowadays are not willing to bow down before an emperor, even a benevolent one, in order to be democratised. They will protest, and the ensuing pain will be felt by the imperial power as well as by its subjects. For Americans, the pain will not be just a matter of budget deficits and body bags; it will also be a blow to the very heart of what makes them American—their constitutional belief in freedom. Freedom is in their blood; it is integral to their sense of themselves. It binds them together as nothing else does, neither ethnicity, nor religion, nor language. And it is rooted in hostility to imperialism—the imperial rule of George III. Americans know that empires lack democratic legitimacy. Indeed, they once had a tea party to prove it.

Some imperialists may be untroubled by such thoughts. Throughout their imperial history, the British, a rather steak-and-kidney sort of people, not much interested in constitutional concepts, would generally fight to defend their own freedom but did not feel obliged to introduce it in their colonies so long as democracy was in prospect for their subjects one distant day. They were helped in this happy procrastination by powerful practical interests (they exported both settlers and capital to their colonies), by a degree of racism, and by a sustained sense of semi-religious mission. And despite the many hardships, those who ran it also had fun with their empire (lots of dressing up with funny hats, playing polo and shooting tigers); and it was a commercial enterprise (“Trade follows the flag,” noted Rhodes).

Little, if any of this applies to Americans. The neocons may have the missionary zeal, but even this is likely to pall in the face of setbacks. There is certainly no zeal to bear the financial burden: Mr Bush's latest budget was drawn up without any money at all for Afghanistan, and the costs are rising in Iraq (to nearly $4 billion a month, just for the soldiery), even as Mr Rumsfeld says more troops may be needed. Unlike most empires of old, the United States is an importer, these days, both of capital and of migrants.

America has changed since September 11th. The new mood allows for more nationalism, more assertiveness, less patience with allies, a greater readiness to go it alone. But there is no appetite to spend a lifetime in a sweaty country in the service of a noble cause. The memories of Vietnam, where every effort to withdraw or hand over to the locals seemed to lead to further entanglement, have not departed. And though the rhetorical heat may now be turned on Iran and Syria, Mr Rumsfeld and his fellow fire-eaters know full well that Americans are not ready for another invasion. Even if, hallelujah, regime change in such countries could be effected peacefully, would the United States really be prepared to shoulder the white man's burden across the Middle East?

It is unlikely, to say the least; the imperial idea is a big exaggeration, like previous fads. It was fashionable, after all, to declare history at a close not so long ago. The new battlegrounds would be markets, said some pundits. Commerce, ideas and information were the weapons of the modern world; military might was for the pterodactyls. To be sure, America is now going through an imperial phase, but this one has more in common with its earlier imperial phases than with the imperial eras of Britain, Byzantium or Rome. If the assertive nationalists and the democratic imperialists have come together over Iraq, that does not mean the administration has signed up for the entire neocon agenda. And as for the foreign-policy pundits, in time they will move on to a new idea.

That does not mean Mr Bush is wrong to think that democracy is the best hope for the world, though it will surely have to take different forms in different places. He is right. But he is also right in disavowing any imperial intentions. America will have to promote its aims some other way, probably by leading multilateral action. Empire is simply not the American way. If the United States has to intervene in places like Afghanistan and Iraq, and then stay on, it will not enjoy the experience. Running the place, it will discover, is nasty and brutish, so it had better also be short. Good or bad, that is not what most people mean by an imperium.