Fool that I am, I got into an argument with some Nazis recently.

It was in the wake of a cancelled right-wing rally in Cork, and maybe 30 or 40 people were gathered outside the venue which had refused to host them. Some of them were pro-life, some were anti-immigrant, and some – like the guys I stupidly engaged – were white supremacists. The one thing which seemed to unite them all was a belief in various different conspiracy theories.

The lads I was arguing with were of differing ages and, I thought, from different parts of the country and social classes. They seemed too to have differing levels of education, but they were of one voice on the common enemy: George Soros, the 88-year-old Hungarian-born investor and philanthropist.

A pale, overweight man in his early 30s and wearing a flat cap, told me “Leo Varadkar wants to bring in one million foreigners by 2040. It’s right there on YouTube. What’s that going to do to native Irish people?”

On his phone, he showed me a YouTube video of Varadkar talking about the population of Ireland increasing by a predicted million people in the next 20 years. My attempts to explain that the Taoiseach was talking about natural population growth, increased life expectancy, and economic expansion, cut no ice. (The comments below that link are predictably hateful, focussing on Varadkar’s sexuality and his ethnicity – and not the Dungarvan part of it.)

“This is all part of a globalist plan to replace indigenous European people,” Flat Cap said. “George Soros is financing all this.”

A middle-aged man with a pony tail squared up to me and growled that he’d “love to cut George Soros’ f*cking throat”. He told me Soros had betrayed his own people and “hunted the Jews during the War”. His friends all nodded in agreement.

When I replied that Soros had been born in 1940, all the phones came out, and Pony Tail and his friends corrected me, triumphantly, that Soros was actually born in 1930. They were right, but when I asked how he could have sold out anyone while still in his teens, they scoffed at my naivety.

Later, online, I found the tiny kernel of truth behind this anti-Semitic fantasy.

When Nazi Germany invaded Hungary in March 1944, George Soros was 13 years old. The Nazis forbade Jewish children from attending school, ordering them instead to report to the Nazi-established Judenrat (Jewish Council).

“The Jewish Council asked the little kids to hand out the deportation notices,” Soros recalled years later. “I was told to go to the Jewish Council. And there I was given these small slips of paper. I took this piece of paper to my father. He instantly recognized it. This was a list of Hungarian Jewish lawyers. He said, ‘You deliver the slips of paper and tell the people that if they report they will be deported’.”

George Soros didn’t return to his job at the Judenrat, and he and his family managed to survive the war by purchasing falsified documents saying they were Christians.

This inconvenient truth hasn’t stopped the spread of this conspiracy theory by the likes of Alex Jones, Donald Trump Junior, Roseanne Barr and James Woods. There seems to be no end to the far-right conspiracy theories that Soros is behind all evil in the world today, from Repeal the Eighth to the Immigrant Caravan, with Breitbart regularly publishing articles blaming him for everything they despise. But then, for most conspiracy theorists, the Jews are seldom too far from the frame.

Adolf Hitler was a big fan of the 'Protocols of the Elders of Zion', a 1903 anti-Semitic forgery which popularised the notion of a worldwide Jewish conspiracy to control press and banking. The historian Nora Levy said “Hitler used the Protocols as a manual in his war to exterminate the Jews”.

After the Holocaust, high-ranking Nazi Erich von dem Bach-Zelewski admitted there never had been a Jüdische Gefahr (Jewish Menace).

“In reality, (Jews) had no organisation at all, not even an information service,” he said. “If they had had some sort of organisation, these people could have been saved by the millions, but instead, they were taken completely by surprise.”

To this day, the theory of a global Zionist conspiracy (overlapping with Holocaust denial) is accepted as an article of faith in much of the Middle East, and by right-wingers everywhere.

David Aaronovitch’s Voodoo Histories: The Role of the Conspiracy Theory in Shaping Modern History (2010) debunks some of the most popular conspiracy theories and asks why well-educated, reasonable people sometimes believe 'perfectly ridiculous things'.

Aaronovitch says key common characteristics of conspiracy theories are that they tend to be politically populist, paranoid and granted a veneer of respectability by selective quotations from often dubiously-qualified academics ('death by footnote') and authentic-sounding jargon like 'false flag', 'wet-works' and 'psy-ops'.

Conspiracy theories usually claim to lay bare the actions of a small power elite. Belief in the conspiracy makes you part of a heroic group who can see past the official version propagated by the powers that be.

Deborah Lipstadt, the American historian who won a libel case taken against her by David Irving, likened arguing with a Holocaust denier to trying to nail jelly to a wall. In the internet era, Professor Nicholas Terry, a history lecturer at Exeter University, says: “It’s now like trying to nail smoke to a wall. There’s almost no substance.”

The problem with arguing with conspiracy theorists is that they are so full of fever-dream certainty, every attempt to expose a crack in the theory only reinforces their belief. Every illogicality revealed serves to strengthen the impregnability of the conspiracy. It’s turtles all the way down.

What my fascist friends in Cork were espousing was 'The Great Replacement' theory, a delusional, paranoid notion that there is a deliberate plan to replace white Europeans with Arab, Middle-Eastern and African people. Who do you think they think might be behind such a scheme?

“Jews will not replace us,” chanted the neo-Nazis who marched on Charlottesville two years ago, Donald Trump’s “very fine people”. Counter-protester Heather Heyer was murdered when white supremacist James Alex Fields Junior drove his car into the crowd. Fields was last year convicted, and sentenced to life imprisonment.

If you think conspiracy theories are just a harmless bit of fun, don’t ever forget that a conspiracy theory helped Adolf Hitler to murder over six million people. And in the face of that horror, his fans turned around and pretended that was a conspiracy theory too.