In Washington these days, people talk a lot about the collapse of the bipartisan foreign policy consensus that existed during the Cold War. But however bitter today’s disputes are about Iraq or the prosecution of the so-called global war on terrorism, there is one bedrock assumption about foreign policy that remains truly bipartisan: The United States will remain the sole superpower, and the guarantor of international security and global trade, for the foreseeable future. In other words, whatever else may change in the decades to come, the 21st century will be every bit as much of an American century as the 20th.

This assumption rests, in turn, on two interrelated beliefs.

The first is that because no country or alliance of states has shown any great desire to challenge U.S. preeminence -- or demonstrated the means of doing so -- no country is going to. China’s interests are regional at most, the argument goes, and the European Union is too divided, too unwilling or too weak to rebuild its once-formidable military machine. As for Russia, believers in the durability of a world order anchored in Washington insist that its declining population and excessive reliance on its energy wealth will in the long run preclude it from playing a central role in global affairs.

The second is that the world needs the U.S. and appreciates the role it plays. (In some versions of this argument, the world needs the U.S. far more than the U.S. needs the world.) If there have been no serious challenges to American hegemony to date, it is asserted, it is because the U.S. provides what are referred to by foreign policy analysts as “global goods”: It maintains political and economic stability around the world, it guarantees a democratic capitalist world order and, by virtue of its unparalleled military strength, it acts as a world policeman of last resort.

Whatever the merits of this case, surely it is significant that it is most often made by U.S. policy analysts and government officials (as well as, to a lesser extent, by British officials). From Pax Romana through Pax Britannica to the current Pax Americana, empires have justified their own power by insisting that they were not simply serving their own interests but rather the common good. Looking back at the British imperial high-water mark of 1900, H.G. Wells wrote that “the sprawling British Empire still maintained a tradition of free trade, equal treatment and open-handedness to all comers round and about the planet.”

Such confidence in Britain’s fundamental benignity as an empire is matched today by figures across the American political spectrum, from Barack Obama to Rudy Giuliani, from the conservative policy analyst Robert Kagan to the liberal academic Michael Mandelbaum. Whatever their other, substantial differences, all seem convinced that the world works best with the United States at the helm, and that without American leadership, the world would soon become more dangerous and anarchic and less prosperous.

Indeed, if they are to be believed, the only serious threat to U.S. hegemony visible anywhere on the horizon is the American people’s potential unwillingness to support their country as it plays this role.

But what if the Americans who hold these beliefs are not, in fact, clear-eyed observers of the world scene stripped of its anti-imperial mystifications? Instead, what if they are people who have fallen for the same self-delusion that the British ruling class entertained before World War I, which was that their empire was so essential to world stability and, at least when compared with the alternatives and with empires past, so just that its hegemony could and would weather all challenges?

It is hardly farfetched to scan the historical record and conclude that self-love and imperialism go together, whether it was the British imperialist Cecil Rhodes insisting that British colonialism in Africa had been “philanthropy plus 5%" or President Bush insisting that it was America’s special mission to spread democracy throughout the world. But what the historical record also shows is that imperial moments are, in fact, fleeting, and that hegemony has a shorter and shorter shelf life. The Roman Empire lasted more than 700 years (more than a millennium if you count the Byzantines); the British Empire lasted a little more than 300 years in India and less than a century in much of Africa. The economic challenges facing the U.S. at least suggest that America’s time as sole superpower could be shorter still.

Americans, who grow up believing in their country’s exceptionalism (which in foreign policy terms often seems to mean not believing that the historical constraints that apply to other nations apply to the U.S.), are not predisposed to believe that American predominance could possibly be coming to an end. And yet it seems more like wishful thinking than rational analysis to believe that the United States -- which in the coming decades will certainly have to adapt to a multipolar world in geo-economic terms, as China and India reoccupy the central place in the global economy that they had 500 years ago -- can continue indefinitely to play a hegemonic role.

The truth is that whether it is imperial Rome, imperial Spain or imperial Britain, economic strength and political strength have always gone together. Because no one denies that the U.S. will decline in comparative terms economically (though it will almost certainly remain one center of the world economy), the only way one can believe that geopolitics will not also become multipolar is to believe that the U.S. is somehow exempt from what seems one of history’s few ironclad laws. And that is not analysis; that is faith.

The war in Iraq has demonstrated the limits of even America’s vaunted military strength -- the one arena in which the U.S. is likely to remain supreme for decades to come. In an era of asymmetric threats, conventional military power is rapidly becoming an anachronistic measure of a country’s strength.

None of this is to say that the U.S. will not continue to be one of the most important powers -- only that its days of first dictating and then guaranteeing the rules are numbered in an era in which it has become a debtor nation. In any case, the post-World War II structures of international governance are crumbling -- as well they might after more than six decades. Everyone knows they need to be revised.

For the moment, the U.S. is the sole superpower. But instead of deluding ourselves that we will go on that way into the indeterminate future, an intelligently self-interested foreign policy would have us do everything in our power to shape, according to our most urgent priorities, the international rules that will govern relations between states after the American moment has passed -- as it inevitably will.

The alternative is to go the route of the British before 1914 and imagine that because a certain set of political arrangements seems best to us, they must also be best for the world -- and destined to endure indefinitely. The real choice that confronts us is not between a second American century and anarchy but between a multipolar world in which we will play an important role and an anti-American century.

David Rieff is the author of many books, including “At the Point of a Gun: Democratic Dreams and Armed Intervention” and “A Bed for the Night: Humanitarianism in Crisis.”