To look back on the ideas that shaped the past decade is to survey a scene of wreckage. Ten years ago, the best and the brightest were believers in the "Washington consensus" - the idea that the debt-fuelled free market that had existed in the US for little more than a decade was the only economic system consistent with the imperatives of modernity, and destined to spread universally.

It was not only the neocon right that believed this. Centre-left parties, whose historical role had been to set limits on free markets, bought in to this idea with enthusiasm. When Bill Clinton and Tony Blair embraced neoliberal economics, they did more than triangulate policies for the sake of electoral advantage. They endorsed the belief that a bubble engineered by Alan Greenspan at the end of the 1990s, when he lowered interest rates to artificial levels after the blow-up of a hedge fund, represented a new era in economic history. Both the triangulating politicians and many left-of-centre commentators became convinced that, for all practical purposes, neoliberal capitalism was indestructible.

For anyone with a sense of history, the idea that a post-cold-war bubble embodied a new world order was obviously absurd. The built-in instability of capitalism had not gone away - it had been accentuated, as the US and other western economies became ever more dependent on unsustainable debt. Far from being in­destructible, the neoliberal market order was highly fragile. But millennial fantasies regarding a short-lived variety of capitalism were far from being the only delusional beliefs that helped shape events during the decade.

Closely related was liberal interventionism - the policy, set out by Tony Blair in his 1999 Chicago speech on foreign policy, of using military force to spread liberal democracy. Here the delusions were multilayered, and first among them was a dream about America. Again, it was not only the right that bought in to a fantasy. For large sections of the left, the US in the first decade of the century had a role similar to that played in the progressive imagination by the USSR in earlier periods: for all its faults, the US was the world's emancipatory power, and the current embodiment of the best human hopes.

The delusive quality of this view lay not so much in the comical notion that universal freedom could be spearheaded by the witless figure of George W Bush, as in an unrealistic estimate of America's position in the world. US imperial overstretch had already been identified in 1987 by Paul Kennedy in his book The Rise and Fall of the Great Powers, but this weakness was forgotten in the triumphalism that surrounded the collapse of the Soviet Union. Not only did the US lack the skills needed to maintain its imperial role, but increasingly it was a role the US could not afford. Both these facts were brutally confirmed in the invasion of Iraq. Confident that liberal democracy would emerge of its own accord once tyranny had been overthrown, the Bush administration was unprepared for the sectarian warfare and near-anarchy that predictably erupted when Saddam Hussein's regime was destroyed. It was equally unprepared for the ruinous cost of the war, which was launched on the assumption that the price of oil would fall after regime change, setting off another global boom and making the entire exercise self-financing. The actual result was that the Americans racked up even more debt and the decline of US power accelerated.

Fashionable theories of globalisation had the effect of blocking the perception of American decline. From the late 1990s onwards, the idea that globalisation and Americanisation were one and the same became something like conventional wisdom, the New York Times correspondent Thomas Friedman pushing the equation in his bestselling books The Lexus and the Olive Tree (1999) and The World Is Flat (2005). The actual effect of globalisation is to transfer economic power to emerging countries with different models of capitalism, but for those who shared Friedman's dreamworld, globalisation was no more than the Anglo-American free market writ large. It was a notion that could persist only so long as the crisis was confined to powerless countries on the periphery of the system, such as Argentina and Thailand. It began to dissipate when the US financial system itself started to implode from mid-2007 onwards. Having run down and sold off much of its productive base, the US found itself the centre of a type of finance capitalism that was practically bankrupt.

It is not often that large-scale crises are due to intellectual error, but a single erroneous belief runs through all of the successive delusions of the past decade. With few exceptions, both left and right seem to think that history is a directional process whose end point - after many unfortunate detours - will be the worldwide duplication of people very like themselves. At the end of the decade, opinion-formers in Britain, the United States and continental Europe still imagine that the normal pattern of historical development leads eventually to an idealised version of western society, just as Francis Fukuyama forecast 20 years ago.

But whereas this confidence-boosting notion was still genuinely believed a decade ago, today it is a kind of comfort blanket against an unfamiliar world. The reality, which is that western power is in retreat nearly everywhere, is insistently denied. Yet the rise of China means more than the emergence of a new great power. Its deeper import is that the ideologies of the past century - neoliberalism just as much as communism - are obsolete. Belief systems in which the categories of western religion are reproduced in the guise of pseudo-science, they are redundant in a world where the most rapidly advancing nation state has never been monotheist. Western societies are well worth defending, but they are not a model for all of humankind. In future they will be only one of several versions of tolerable modernity.

For secular western intellectuals to accept this fact would rob their life of meaning. Huddled in the tattered blanket of historical teleology, which tells them they are the leading lights of humanity, they screen out any development that demonstrates their increasing irrelevance. Religion is resurgent in many parts of the world, not least emerging powers such as Brazil and China, but for the secular intelligentsia this is just an unfortunate lag, a temporary setback in humanity's slow march to join them on the sunlit uplands of reason. The hysterical stridency of evangelical atheism - one of the most characteristic phenomena of the Noughties - is symptomatic of a pervasive cognitive dissonance. Like everyone else, these intellectuals assert their beliefs all the more adamantly when the only reason for holding them is a well-founded suspicion that they are not true.

Unreality is the defining feature of the ideas that have been in vogue over the past decade. The grandiose delusions with which the new century began have not been abandoned. Instead, they have been shrunk to a size at which they can still be maintained. The small world of British politics provides many examples of this tendency. Rather than acknowledge that neoliberalism has failed, politicians in all three main parties are seizing on a succession of intellectual gimmicks for solutions to the problems that the ideology has created. Gladwell's blink, Sunstein and Thaler's nudge, the wisdom of crowds - these and other ephemera of the airport bookstore are being taken up, promoted and then forgotten in the floundering attempt to deal with a crisis that is only in its early stages.

The intellectual default of politicians cannot be remedied by returning to the ideologies of the past. It is shared by much of the public, and comes from a chronic inability to engage with reality. Perhaps only a more serious crisis will overturn the delusive fancies on which so many policies are based. A run on sterling in the event of a hung parliament after the next general election; the cataclysmic defeat that will follow Barack Obama's decision to reinforce inevitable failure in Afghanistan; a spiral in oil prices after a flare-up over Iran; the collapse of the dollar as the world finally loses patience with American solipsism - any one of these eventualities, together with others that cannot be foreseen, could be a catalyst for rethinking.

But the omens are not encouraging. The make-believe that surrounds climate change - epitomised in the empty statements of intent regarding unachievable goals that will be the only outcome of the Copenhagen meeting - shows that the biggest challenge for the future is being evaded. It looks as if we may be wandering in the ruins of the Noughties for some time.

John Gray is the New Statesman's lead book reviewer. A new edition of his "False Dawn: the Delusions of Global Capitalism", which first appeared in 1998, was published by Granta Books in October (£8.99)

The ideas that shaped a decade

Neoliberalism: Three policies central to the neoliberal "Washington consensus" were low taxation, privatisation and the deregulation of financial services. Key thinkers: Friedrich Hayek, Milton Friedman, Robert Nozick.

Neoconservatism: The term was originally applied to disillusioned liberal critics of the welfare state. By the beginning of the 21st century, neoconservatism was associated principally with an aggressive US foreign policy. Key thinkers: Henry "Scoop" Jackson, Leo Strauss.

Political Islam: "Islamism", or political Islam, is dominated by two distinct and extreme strands of thought: the Salafist or Saudi Wahhabi tradition; and the work of Sayyid Qutb, who saw Islam as a political movement based on Quranic principles and from whom Osama Bin Laden derived the doctrine of violent jihad. Key thinkers: Sayyid Qutb, Mohammad Ibn Abdul Wahhab, Ayman al-Zawahiri.