Since the attacks on the United States in September 2001, there have been more than a thousand references in British national newspapers, working out at almost one every single day, to the phrase "dirty bomb". There have been articles about how such a device can use ordinary explosives to spread lethal radiation; about how London would be evacuated in the event of such a detonation; about the Home Secretary David Blunkett's statement on terrorism in November 2002 that specifically raised the possibility of a dirty bomb being planted in Britain; and about the arrests of several groups of people, the latest only last month, for allegedly plotting exactly that.

Starting next Wednesday, BBC2 is to broadcast a three-part documentary series that will add further to what could be called the dirty bomb genre. But, as its title suggests, The Power of Nightmares: The Rise of the Politics of Fear takes a different view of the weapon's potential.

"I don't think it would kill anybody," says Dr Theodore Rockwell, an authority on radiation, in an interview for the series. "You'll have trouble finding a serious report that would claim otherwise." The American department of energy, Rockwell continues, has simulated a dirty bomb explosion, "and they calculated that the most exposed individual would get a fairly high dose [of radiation], not life-threatening." And even this minor threat is open to question. The test assumed that no one fled the explosion for one year.

During the three years in which the "war on terror" has been waged, high-profile challenges to its assumptions have been rare. The sheer number of incidents and warnings connected or attributed to the war has left little room, it seems, for heretical thoughts. In this context, the central theme of The Power of Nightmares is riskily counter-intuitive and provocative. Much of the currently perceived threat from international terrorism, the series argues, "is a fantasy that has been exaggerated and distorted by politicians. It is a dark illusion that has spread unquestioned through governments around the world, the security services, and the international media." The series' explanation for this is even bolder: "In an age when all the grand ideas have lost credibility, fear of a phantom enemy is all the politicians have left to maintain their power."

Adam Curtis, who wrote and produced the series, acknowledges the difficulty of saying such things now. "If a bomb goes off, the fear I have is that everyone will say, 'You're completely wrong,' even if the incident doesn't touch my argument. This shows the way we have all become trapped, the way even I have become trapped by a fear that is completely irrational."

So controversial is the tone of his series, that trailers for it were not broadcast last weekend because of the killing of Kenneth Bigley. At the BBC, Curtis freely admits, there are "anxieties". But there is also enthusiasm for the programmes, in part thanks to his reputation. Over the past dozen years, via similarly ambitious documentary series such as Pandora's Box, The Mayfair Set and The Century of the Self, Curtis has established himself as perhaps the most acclaimed maker of serious television programmes in Britain. His trademarks are long research, the revelatory use of archive footage, telling interviews, and smooth, insistent voiceovers concerned with the unnoticed deeper currents of recent history, narrated by Curtis himself in tones that combine traditional BBC authority with something more modern and sceptical: "I want to try to make people look at things they think they know about in a new way."

The Power of Nightmares seeks to overturn much of what is widely believed about Osama bin Laden and al-Qaida. The latter, it argues, is not an organised international network. It does not have members or a leader. It does not have "sleeper cells". It does not have an overall strategy. In fact, it barely exists at all, except as an idea about cleansing a corrupt world through religious violence.

Curtis' evidence for these assertions is not easily dismissed. He tells the story of Islamism, or the desire to establish Islam as an unbreakable political framework, as half a century of mostly failed, short-lived revolutions and spectacular but politically ineffective terrorism. Curtis points out that al-Qaida did not even have a name until early 2001, when the American government decided to prosecute Bin Laden in his absence and had to use anti-Mafia laws that required the existence of a named criminal organisation.

Curtis also cites the Home Office's own statistics for arrests and convictions of suspected terrorists since September 11 2001. Of the 664 people detained up to the end of last month, only 17 have been found guilty. Of these, the majority were Irish Republicans, Sikh militants or members of other groups with no connection to Islamist terrorism. Nobody has been convicted who is a proven member of al-Qaida.

In fact, Curtis is not alone in wondering about all this. Quietly but increasingly, other observers of the war on terror have been having similar doubts. "The grand concept of the war has not succeeded," says Jonathan Eyal, director of the British military thinktank the Royal United Services Institute. "In purely military terms, it has been an inconclusive war ... a rather haphazard operation. Al-Qaida managed the most spectacular attack, but clearly it is also being sustained by the way that we rather cavalierly stick the name al-Qaida on Iraq, Indonesia, the Philippines. There is a long tradition that if you divert all your resources to a threat, then you exaggerate it."

Bill Durodie, director of the international centre for security analysis at King's College London, says: "The reality [of the al-Qaida threat to the west] has been essentially a one-off. There has been one incident in the developed world since 9/11 [the Madrid bombings]. There's no real evidence that all these groups are connected." Crispin Black, a senior government intelligence analyst until 2002, is more cautious but admits the terrorist threat presented by politicians and the media is "out of date and too one-dimensional. We think there is a bit of a gulf between the terrorists' ambition and their ability to pull it off."

Terrorism, by definition, depends on an element of bluff. Yet ever since terrorists in the modern sense of the term (the word terrorism was actually coined to describe the strategy of a government, the authoritarian French revolutionary regime of the 1790s) began to assassinate politicians and then members of the public during the 19th century, states have habitually overreacted. Adam Roberts, professor of international relations at Oxford, says that governments often believe struggles with terrorists "to be of absolute cosmic significance", and that therefore "anything goes" when it comes to winning. The historian Linda Colley adds: "States and their rulers expect to monopolise violence, and that is why they react so virulently to terrorism."

Britain may also be particularly sensitive to foreign infiltrators, fifth columnists and related menaces. In spite, or perhaps because of, the absence of an actual invasion for many centuries, British history is marked by frequent panics about the arrival of Spanish raiding parties, French revolutionary agitators, anarchists, bolsheviks and Irish terrorists. "These kind of panics rarely happen without some sort of cause," says Colley. "But politicians make the most of them."

They are not the only ones who find opportunities. "Almost no one questions this myth about al-Qaida because so many people have got an interest in keeping it alive," says Curtis. He cites the suspiciously circular relationship between the security services and much of the media since September 2001: the way in which official briefings about terrorism, often unverified or unverifiable by journalists, have become dramatic press stories which - in a jittery media-driven democracy - have prompted further briefings and further stories. Few of these ominous announcements are retracted if they turn out to be baseless: "There is no fact-checking about al-Qaida."

In one sense, of course, Curtis himself is part of the al-Qaida industry. The Power of Nightmares began as an investigation of something else, the rise of modern American conservatism. Curtis was interested in Leo Strauss, a political philosopher at the university of Chicago in the 50s who rejected the liberalism of postwar America as amoral and who thought that the country could be rescued by a revived belief in America's unique role to battle evil in the world. Strauss's certainty and his emphasis on the use of grand myths as a higher form of political propaganda created a group of influential disciples such as Paul Wolfowitz, now the US deputy defence secretary. They came to prominence by talking up the Russian threat during the cold war and have applied a similar strategy in the war on terror.

As Curtis traced the rise of the "Straussians", he came to a conclusion that would form the basis for The Power of Nightmares. Straussian conservatism had a previously unsuspected amount in common with Islamism: from origins in the 50s, to a formative belief that liberalism was the enemy, to an actual period of Islamist-Straussian collaboration against the Soviet Union during the war in Afghanistan in the 80s (both movements have proved adept at finding new foes to keep them going). Although the Islamists and the Straussians have fallen out since then, as the attacks on America in 2001 graphically demonstrated, they are in another way, Curtis concludes, collaborating still: in sustaining the "fantasy" of the war on terror.

Some may find all this difficult to swallow. But Curtis insists,"There is no way that I'm trying to be controversial just for the sake of it." Neither is he trying to be an anti-conservative polemicist like Michael Moore: "[Moore's] purpose is avowedly political. My hope is that you won't be able to tell what my politics are." For all the dizzying ideas and visual jolts and black jokes in his programmes, Curtis describes his intentions in sober, civic-minded terms. "If you go back into history and plod through it, the myth falls away. You see that these aren't terrifying new monsters. It's drawing the poison of the fear."

But whatever the reception of the series, this fear could be around for a while. It took the British government decades to dismantle the draconian laws it passed against French revolutionary infiltrators; the cold war was sustained for almost half a century without Russia invading the west, or even conclusive evidence that it ever intended to. "The archives have been opened," says the cold war historian David Caute, "but they don't bring evidence to bear on this." And the danger from Islamist terrorists, whatever its scale, is concrete. A sceptical observer of the war on terror in the British security services says: "All they need is a big bomb every 18 months to keep this going."

The war on terror already has a hold on western political culture. "After a 300-year debate between freedom of the individual and protection of society, the protection of society seems to be the only priority," says Eyal. Black agrees: "We are probably moving to a point in the UK where national security becomes the electoral question."

Some critics of this situation see our striking susceptibility during the 90s to other anxieties - the millennium bug, MMR, genetically modified food - as a sort of dress rehearsal for the war on terror. The press became accustomed to publishing scare stories and not retracting them; politicians became accustomed to responding to supposed threats rather than questioning them; the public became accustomed to the idea that some sort of apocalypse might be just around the corner. "Insecurity is the key driving concept of our times," says Durodie. "Politicians have packaged themselves as risk managers. There is also a demand from below for protection." The real reason for this insecurity, he argues, is the decay of the 20th century's political belief systems and social structures: people have been left "disconnected" and "fearful".

Yet the notion that "security politics" is the perfect instrument for every ambitious politician from Blunkett to Wolfowitz also has its weaknesses. The fears of the public, in Britain at least, are actually quite erratic: when the opinion pollsters Mori asked people what they felt was the most important political issue, the figure for "defence and foreign affairs" leapt from 2% to 60% after the attacks of September 2001, yet by January 2002 had fallen back almost to its earlier level. And then there are the twin risks that the terrors politicians warn of will either not materialise or will materialise all too brutally, and in both cases the politicians will be blamed. "This is a very rickety platform from which to build up a political career," says Eyal. He sees the war on terror as a hurried improvisation rather than some grand Straussian strategy: "In democracies, in order to galvanize the public for war, you have to make the enemy bigger, uglier and more menacing."

Afterwards, I look at a website for a well-connected American foreign policy lobbying group called the Committee on the Present Danger. The committee features in The Power of Nightmares as a vehicle for alarmist Straussian propaganda during the cold war. After the Soviet collapse, as the website puts it, "The mission of the committee was considered complete." But then the website goes on: "Today radical Islamists threaten the safety of the American people. Like the cold war, securing our freedom is a long-term struggle. The road to victory begins ... "

· The Power of Nightmares starts on BBC2 at 9pm on Wednesday October 20.