Although I present myself as an open-minded chap, I can remember very few times when I've admitted being in the wrong. Not wrong in detail, but wrong in principle. In my experience the politically committed rarely do that. We change imperceptibly and grudgingly, while all the time pretending we haven't changed at all but merely adapted to altered circumstances. Actually, 'very few' is a self-serving exaggeration. The only time I realised I was charging up a blind alley was when I read Paul Berman's Terror and Liberalism. I didn't see a blinding light or cry 'Eureka!' If I was going to cry anything it would have been 'Oh bloody hell!'. Berman convinced me I'd wasted a great deal of time looking through the wrong end of the telescope. I was going to have to turn it round and see the world afresh. The labour would involve reconsidering everything I'd written since 11 September 2001, arguing with people I took to be friends and finding myself on the same side as people I took to be enemies. All because of Berman.

The bastard.

At the time, I was trying to write a book of my own, Pretty Straight Guys, which was an assault on the sliminess of New Labour and the millennial madness of the dotcom bubble. I was aware I'd have to tackle the left - Why did it always lose? Why didn't it have solutions anyone on the planet could use? I was working at the Observer and the New Statesman and had noticed already a sour smell in the air. People were very edgy about religion. Not Christianity or Judaism, naturally, but Islam, or rather Islamism. Its loathing of democracy and human rights, its hatred of women's emancipation and its glorification of suicide and murder was never described for what it was: a pathological movement of the far right.

Then there was Iraq. I'd just finished a chapter on the loathsome way asylum seekers were treated by Blair, and had to admit that an awful lot of the mistreated were on the run from Saddam Hussein. But all around me Iraqis were vanishing from polite society's conversation. The Kurdish socialists, who had received liberal London's tea and sympathy when Saddam was America's ally, had been forgotten. Saddam's murders of Kurds and Arabs became, by a greasy process which was never made explicit, the fault of Britain and America. By 2003, the tyrant of Iraq was held to be no longer responsible for crimes committed in the tyranny of Iraq.

I'd have probably made a few mild comments and got into fewer rows if Terror and Liberalism had not landed on my desk. It's a dangerously seductive book whose central point is that Islamism and Ba'athism are continuations of Nazism and communism, not only in their fine points - the founders of the Muslim Brotherhood and Ba'ath Party were admirers of Hitler and Franco - but in their fundamentals. Once again we had the promise of earthly paradise, but not now the paradise of unexploited labour or of an Aryan Europe, but the paradise of the early days of the prophet or a reunified Arab nation, pure and free. Once again there were semi-divine leaders who led the faithful into cosmic struggles. And once again their programmes were insane.

Cleverly, Berman treats his targets very sympathetically. Readers who want to disagree with him, as I did, are charmed because he understands why we believed what we believe and more often than not explains our ideas better than we can. The Russian terrorists who began the violence which finished with the slaughters of the communists are morally scrupulous. They won't throw a bomb into the coach of the Grand Duke Sergei, because there are children on board, or risk the lives of adult non-combatants. True, they have no ideas beyond death, their own and others, no plan for society which could possibly succeed, but that doesn't hide the desperation which had driven sensitive and high-minded young men and women to rebel. Similarly, Berman is so angry about the collapse of European civilisation into the barbarism of the World War I that you could imagine him joining the communist party or becoming a Nazi, and is so sympathetic to the intellectual currents buffeting Sayyid Qutb that Qutb's transformation into the intellectual founder of a cult of death appears the most natural of developments, one you might make yourself in the circumstances.

He avoids the usual polemical style because he is trying to overcome the resistance of liberals who have seen the Iranian revolution and the murder of millions, and the enslavement of whole African tribes in the Sudan, and the destruction of every last remnant of freedom in Afghanistan, but not understood that what they've seen is a totalitarian movement going about its business.

A chapter - 'Wishful Thinking' - explains how decent people can end up on the far right by using the history of the French Socialist Party in the 1930s as a parable for our time. Leon Blum, its leader, knew that the Nazis had to be fought. But a large faction, supported by the teachers' unions and many left-wing intellectuals, was horrified by the prospect of a conflict which could exceed the carnage of World War I.

If they had looked the Nazis in the face, they would have realised that war was inevitable. Rather than see clearly, they allowed the best of motives to convince them that the German people hadn't fallen for an insane cult. Why would they? Wasn't it almost racist to believe that they were anything other than as rational and decent as the French?

Take Hitler's demands to expand the German Reich. In a certain light these could be seen as a menacing expansion of the Nazi state, but was it not the case that the Treaty of Versailles had imposed punitive conditions on Germany at the end of the World War I? Was it not reasonable for Hitler to ask that Germans should be freed from control by the Poles and the Czechs and returned to their mother country? Hitler may have been from the extreme right and they may have been from the democratic left, but an argument wasn't necessarily wrong just because Hitler made it.

Many socialists were therefore enthusiastic supporters of the Munich agreement. They believed, says Berman, in the 'simple-minded optimism' of 19th century liberalism - a liberalism of denial. Human beings were essentially rational. Politicians and propagandists who pretended otherwise were the tools of the arms corporations and media empires which were leading France into an unnecessary pre-emptive war:

"The anti-war socialists," writes Berman, "gazed across the Rhine and simply refused to believe that millions of upstanding Germans had enlisted in a political movement whose animating principles were paranoid conspiracy theories, blood-curdling hatreds, mediaeval superstitions and the lure of murder. At Auschwitz the SS said "Here there is no why." The anti-war socialists in France believed no such thing. In their eyes, there was always a why."
There was a price to pay for rationalism. Obviously, the socialists couldn't begin to show solidarity with the German socialists who were being persecuted by Hitler. How could they protest at their treatment or organise parliamentary debates calling attention to their plight when they were making excuses for the Hitler who was doing the persecuting? Then there were the Nazis' Jewish victims. As good men and women of the Enlightenment, the anti-war socialists couldn't tolerate anti-Semitism. Yet they were determined not to let their sympathies get out of hand. Weren't the Jews always showing their wounds and trying to make others feel guilty for their past suffering? Hitler might be going a bit far, but wasn't it true that a disproportionate number of industrialists and financiers were Jewish? And wasn't it also the case that their leader, Leon Blum, who was urging France to enter a bloody and worthless confrontation with Germany was, well, Jewish, too?

In 1940, Hitler gave irrefutable proof of his intentions when he invaded France. The French extreme right under the leadership of Marshall Pétain proposed a collaborationist government. Blum and some socialists chose to fight, but many of their colleagues accepted the occupation and, as Berman concludes, went the whole hog and joined the Vichy government.

Some of these socialists went a little further too, and began to see a virtue in Pétain's programme for a new France and a new Europe: " a programme of strength and vitality, a Europe ruled by a single-party state instead of by the corrupt cliques of bourgeois democracy, a Europe cleansed of the impurities of Judaism and of the Jews themselves, a Europe of the anti-liberal imagination. And in that very remarkable fashion, a number of the anti-war socialists of France came full circle. They had begun as defenders of liberal values and human rights, and they evolved into the defenders of bigotry, tyranny, superstition and mass murder. They were democratic leftists who, through the miraculous workings of the slippery slope and a naïve rationalism, of all things, ended as fascists. Long ago, you say? Not so long ago."

Indeed not. To see the old process at work, you only have to look at how a large chunk of liberal opinion has got itself into the position where it can't support Iraqi and Afghan liberals, socialists and feminists. You think the worst thing in the world is the developed countries, which do, to be fair, have manifold faults. You are confronted with totalitarian movements, which are worse, and your first thought is to blame them on the West. Your second is to make excuses for them. Your third is to betray your comrades. Your fourth is to go up to the totalitarian movements and shake them by the hand.

Does the realisation that there are worse things in the world than George W Bush and Tony Blair expel you from the left? Maybe. I could say that I'm one of the few journalists in the liberal press who can pick up the phone and talk to an Iraqi communist or a Kurdish socialist as a comrade, but that's ducking the issue. If most who say they're on the left prefer anti-capitalism to anti-fascism, then you aren't of the left.

But so what? As Islamists bombs blow to pieces democrats in Iraq and commuters in London, there are more pressing matters than political positioning.

The original version of this article appeared on normblog Nick Cohen's Pretty Straight Guys is out now in paperback