It would have been a lot neater, and no doubt a lot more comforting, if Anders Behring Breivik had been declared too insane to stand trial. The survivors of his massacre would have been spared the sight of Breivik saluting the TV cameras on his way into court and he would not have been able to use the international attention to promote the doctrine that he claims justified the killings.

In a short film played to the court on the first day of the trial, Breivik set out his theory that western civilisation was under attack from multiculturalism, an “anti-European hate ideology” orchestrated by “cultural Marxists”, who had encouraged the Islamic “colonisation” of Europe in order to destroy traditional Christian values.

Taken in isolation, his views do seem like a paranoid delusion – and that is perhaps why an initial psychiatric report declared Breivik to be suffering from schizophrenia. Yet if the beliefs he claims to hold really are delusional, then the frightening thing is that they did not spring forth from a single, deranged mind: they represent a far-right ideology shared by groups across Europe and the US.

Breivik claimed to be part of the “counter-jihad” movement, a network of bloggers and political activists who believe that Muslim immigrants threaten not only violence but “demographic jihad”, simply by living here and having children. These ideas have inspired a new wave of far-right movements, chief among them being the English Defence League.

The leaders of this street protest group, which emerged in 2009, are Breivik’s ideological cousins: its principal spokesman, Stephen Yaxley-Lennon (who goes by the pseudonym “Tommy Robinson”), has distanced himself from Breivik’s methods but was quoted in an interview praising his “cunning”. Last year, in the aftermath of the Norway killings, Yaxley-Lennon predicted similar events in Britain if people did not “listen” to the EDL.

Dark origins

The “cultural Marxism” that Breivik blamed for Europe’s Muslim takeover is a conspiracy theory that was born in the US. It contends that a small group of Marxist philosophers associated with the Frankfurt school of critical theory plotted to destroy western civilisation by encouraging multiculturalism, homosexuality and collectivist economic ideas.

Although many don’t realise it today, the theory is anti-Semitic in origin and its early proponents emphasised that these philosophers were all Jewish. Breivik’s lengthy “manifesto” devotes an entire section to profiling Theodor Adorno, Herbert Marcuse and other Frankfurt school thinkers.

A threat to ethnic purity; betrayal by corrupt elites; the presence of a foreign invader – these are familiar themes for the far right. But the ideology of the “counter-jihad” movement marks a shift from neo-Nazism, whose followers believe above all in the international Jewish conspiracy – and that immigration is a Jewish-led plot to dilute European racial stock.

The difference here is that Breivik’s themes have widespread mainstream credibility. Islamophobia is rampant across western Europe, while Britain’s press leads the field with its drip-feed of anti-Muslim coverage.

Even the idea of “cultural Marxism” has found its way into the mainstream, dovetailing with right-wing ideologues who would have us believe that liberal elites have foisted their agenda on an unwilling population. In the US, it was promoted by the likes of the late commentator Andrew Breitbart, while here it has been echoed by conservatives. Last September, the writer James Delingpole claimed that the BBC had fallen victim to a Marxist “plan to destroy western civilisation from within”. Earlier this month, a Daily Mail blogger even suggested that the New Statesman’s founders, Beatrice and Sidney Webb, were dedicated to the “destruction of traditional western civilisation” and that the London School of Economics, which they also founded, was a nest of Frankfurt-style subversion.

To think that every cultural conservative is a secret extremist or a killer-in-waiting would be another kind of paranoid fantasy. But the point about far-right ideology is that it is parasitical on the mainstream.

The fascism of the 1920s and 1930s succeeded because it played on wider fears, winning the support of those who would never have thought of themselves as “extremists”. The Nazis used anti-Semitism because it already existed in German society. Their successors today use Islamophobia because it already exists in our societies. From a tiny grain of truth – the existence of Islamist terror – has been spun a whole mythology about the imminent collapse of western civilisation and, whether they realise it or not, conservative ideologues are helping spread the poison that enables the far right to grow.

Daniel Trilling’s “Bloody Nasty People: the Rise of Britain’s Far Right” will be published by Verso in September