By Paul Reynolds
World affairs correspondent, BBC News website
The financial crisis is likely to diminish the status of the United States as the world's only superpower.
Will Uncle Sam still bestride the world in future?
On the practical level, the US is already stretched militarily, in Afghanistan and Iraq, and is now stretched financially.
On the philosophical level, it will be harder for it to argue in favour of its free market ideas, if its own markets have collapsed.
Some see this as a pivotal moment.
The political philosopher John Gray, who recently retired as a professor at the London School of Economics, wrote in the London paper The Observer: "Here is a historic geopolitical shift, in which the balance of power in the world is being altered irrevocably.
"The era of American global leadership, reaching back to the Second World War, is over... The American free-market creed has self-destructed while countries that retained overall control of markets have been vindicated."
"In a change as far-reaching in its implications as the fall of the Soviet Union, an entire model of government and the economy has collapsed.
"How symbolic that Chinese astronauts take a spacewalk while the US Treasury Secretary is on his knees."
No apocalypse now
Not all would agree that an American apocalypse has arrived. After all, the system has been tested before.
John Bolton gives rumours of US hegemony's demise short shrift
In 1987 the Dow Jones share index fell by more than 20% in one day. In 2000, the dot-com bubble burst. Yet both times, the US picked itself up, as it did post Vietnam.
Prof Gray's comments certainly did not impress one of the more hawkish figures who served in the Bush administration, the former UN ambassador John Bolton.
When I put them to him, he replied only: "If Professor Gray believes this, can he assure us that he is selling his US assets short?
"If so, where is he placing his money instead? And if he has no US assets, why should we be paying any attention to him?"
Nevertheless, it does seem that the concept of the single superpower left bestriding the world after the collapse of communism (and the supposed end of history) is no longer valid.
Even leading neo-conservative thinkers accept that a more multi-polar world is emerging, though one in which they want the American position to be the leading one.
Robert Kagan, co-founder in 1997 of the "Project for the New American Century" that called for "American global leadership", wrote in Foreign Affairs magazine this autumn: "Those who today proclaim that the United States is in decline often imagine a past in which the world danced to an Olympian America's tune. That is an illusion.
The US is seen as declining relatively and there has been an enormous acceleration in this perfect storm of perception in the waning days of the Bush administration
Dr Robin Niblett
"The world today looks more like that of the 19th Century than like that of the late 20th.
"Those who imagine this is good news should recall that the 19th Century order did not end as well as the Cold War did."
"To avoid such a fate, the United States and other democratic nations will need to take a more enlightened and generous view of their interests than they did even during the Cold War. The United States, as the strongest democracy, should not oppose but welcome a world of pooled and diminished national sovereignty.
"At the same time, the democracies of Asia and Europe need to rediscover that progress toward this more perfect liberal order depends not only on law and popular will but also on powerful nations that can support and defend it."
The director of a leading British think-tank Chatham House, Dr Robin Niblett, who has worked on both sides of the Atlantic, remarked that, at a recent conference he attended in Berlin, an American who called for continued US leadership was met with a new scepticism.
Despite its feats in space, China is said to face a future food crisis
"The US is seen as declining relatively and there has been an enormous acceleration in this perfect storm of perception in the waning days of the Bush administration. The rise of new powers, the increase in oil wealth among some countries and the spread of economic power around the world adds to this," he said.
"But we must separate the immediate moment from the structural. There is no doubt that President Bush has created some of his own problems. The overstretch of military power and the economic crisis can be laid at the door of the administration.
"Its tax cuts were not matched by the hammer of spending cuts. The combined effect of events like the failures in Iraq, the difficulties in Afghanistan, the thumbing of its nose by Russia in Georgia and elsewhere, all these lead to a sense of an end of an era.
The longer term
Dr Niblett argues that we should wait a bit before coming to a judgment and that structurally the United States is still strong.
America has been stretched by wars in Iraq and Afghanistan
"America is still immensely attractive to skilled immigrants and is still capable of producing a Microsoft or a Google," he went on.
"Even its debt can be overcome. It has enormous resilience economically at a local and entrepreneurial level.
"And one must ask, decline relative to who? China is in a desperate race for growth to feed its population and avert unrest in 15 to 20 years. Russia is not exactly a paper tiger but it is stretching its own limits with a new strategy built on a flimsy base. India has huge internal contradictions. Europe has usually proved unable to jump out of the doldrums as dynamically as the US.
"But the US must regain its financial footing and the extent to which it does so will also determine its military capacity. If it has less money, it will have fewer forces."
With the US presidential election looming, it will be worth returning to this subject in a year's time to see how the world, and the American place in it, looks then.