There are, as Americans sometimes remember to remind themselves, two Americas. There is rich America and there is poor America; America of the have-so-much, and America of the have-nots. There is bold, noisy America that insists on ever reducing the size of government; and there is timid America that seeks a way to provide health insurance for its 50 million citizens who have none.

There is the America of opinionated clamour - of Bill O'Reilly's reduction of public figures within a rude dichotomy of "pinheads and patriots", and of CNN's Lou Dobbs's nightly rants on illegal immigration. And then there is the America that offers up Cullen Murphy.

Murphy, for 21 years until 2006 the managing editor of The Atlantic Monthly, and now Vanity Fair editor-at-large, is author of The New Rome?, a book-length essay that asks whether the world's sole remaining superpower must go the way of all empires before it.

His analysis of the country and his policy prescriptions for it are so much at odds with the popular currency of political debate as it seeks a successor to George Bush, that it is surprising the book has been so positively received.

He urges acceptance of those illegal migrants that so stir Dobbs, a rethinking of energy policy to lessen reliance on the military to protect US interests, an insistence that the atrophy of government cease so it can do well those things to which it is suited, and a reversal of the decades-old practice of privatisation of the business of government.

"Many of the things I am getting at here go beyond Rome and America," Murphy says. "In some respects [the book] is an exercise in how you think about social change, and Rome and America happen to be two sticks you can rub together to make a fire.

"What it's really looking at is what does human nature do to institutions over time, how the best laid plans go awry, how any large system becomes untenable over time. These are issues that are not time-bound, and they are not culture-bound either." Much of the book was researched and written within Boston's 200-year-old Athenaeum library, as fitting a place as could be imagined for an exercise in historical reflection.

Iron-railed mezzanine book galleries overlook its reading rooms, which burst with classical busts and 18th- and 19th-century oil colours. Several of its rooms offer views of an adjoining 17th-century burial ground which includes among its notable residents the Revolutionary hero Paul Revere and the father of scientist and philosopher Benjamin Franklin.

Since Rome has always weighed on the American consciousness, from Franklin's time in the earliest days of the republic, this library's place in Murphy's project could hardly be more fitting.

It is not that Murphy believes the two societies are so much alike that their experiences can be interchanged, but he argues that they share certain key habits of thought and behaviour and status. And perhaps it is Rome's function for Americans to act as a reminder of impermanence and of greatness brought undone.

To begin, he ticks off the differences between his subjects: "People overlook the fact that the US is a democracy and Rome was in many ways a hideous autocracy.

"People overlook the fact that Rome was a slave state and that the US put slavery behind it, at great cost, but it did so. That was a huge difference both economically and psychologically.

"Rome had nothing like what we would call a middle class. The social structure was incomparably different from ours. For us the middle class is the essential social fact, as it is for all Western democracies.

"The mental states were different. The self-satisfaction of the Roman elite which I don't think - certainly you find plutocrats and the lazy rich in this country … but I don't think it's fundamentally an American trait to be lazy.

"The characteristic American trait is to be dissatisfied, to be self-critical, to be trying to improve yourself and the world around you, almost to a ludicrous degree. That is a huge difference." Intellectually, they are different also, since Americans, who appear to have no shame and will emote in public almost for practice, have no match for the Roman concept of honour.

Yet the similarities he finds equally striking, and Murphy is unremittingly clear-sighted in his assessments.

He identifies a shared self-importance and self-centredness about ancient Rome and modern America, that is accompanied by their almost wilful ignorance of the rest of the world.

And while holding a raft of stereotypes about people they don't know, such as Arabs, Africans and Germans - "We can produce stereotypes while on autopilot," Murphy writes - Americans also strangely presume that the rest of the world is much like them, or would choose to be but for the misfortune of being born elsewhere.

There is also between the two empires the common experience of a growing divide between military and civil society. Indeed, one of the criticisms often heard in the US now is that the burden of the occupation of Iraq is so unfairly borne by a minority, the military families, who worry and mourn while the rest of the country parties on and lusts over the innovation of high definition television.

Frequently certain Americans tell themselves that their nation only reluctantly resorts to military action, and that theirs is a nation that does not covet others' territory: that the word "empire" has no application for the US. Whereas Murphy will point to the great sweep of the eastern colonies to the south and west that created the continental US to suggest that "empire is in America's DNA".

A further parallel is the sheer size and complexity of imperial Rome and American interests and the constant confrontation this brings. "The bigger the entity and the more things it touches the more susceptible it is to forces beyond its control," he writes. Everything in the world affects a superpower.

Perhaps strongest of Murphy's parallels and of the explicit criticism of his country's path lies in its loss of faith in government and its adoption of widespread privatisation.

When public functions rely on private resources, the lines of responsibility blur and become non-existent. It is here where study of the failure of Rome may be most illuminating, Murphy suggests, because the effects of privatisation on government's ability to manage take so long to manifest themselves.

"It took a long time to happen, but the fraying connection between imperial will and concrete action is a big part of what went wrong in ancient Rome," he writes.

Not so long in coming for the US, however, was the controversy over the strafing of a bus and the killing of 17 Iraqi civilians by private security guards employed by the Blackwater company.

"It was very interesting in the aftermath of that: everybody acknowledged what had happened was terrible, but who was responsible for it?" Murphy says. "The State Department evidently had indemnified Blackwater. The Iraqi Government knew it really did not have control over this organisation. The State Department itself also didn't seem to have control.

"The rules of engagement [Blackwater] was following were not the same rules the military had. It is this grey area that is created when private actors are playing by different rules, and there's no linear sense of authority from the top to the bottom. That to my mind is a truly corrosive phenomenon.

"If you imagine that state of affairs happening in every sector of life, and continuing over a century … but we don't see or think in centuries. We think we're farsighted greybeards if we think in decades." Of ancient Rome, Murphy traces the development of a system of patronage and the buying and selling of influence that he says "hollows out government".