New York’s Madison Square Garden is these days synonymous with American entertainment, offering nothing more sinister than rock concerts, dog shows and a home to the city’s beloved Rangers and Knicks.

But, 80 years ago, the world’s most famous arena played host to a rather darker, more infamous, gathering.

On February 20, 1939, to celebrate the anniversary of George Washington’s birth, over 20,000 American Nazis crammed inside to stage their very own Nuremberg rally.

Quite consciously, they had chosen America’s most Jewish city to parade their support for the Nazis — barely two months after the horrors of Kristallnacht.

With a huge portrait of Washington looking on and a stage bedecked in stars and stripes, the crowd listened to a rendition of The Star Spangled Banner and recited the Pledge of Allegiance.

But there were also swastika banners and the arena echoed to the roar of “Sieg Heil” chants, while outstretched arms greeted a parade of brownshirts down the central aisle.

The rally was the brainchild of Fritz Kuhn, a German-born migrant to the United States and former Ford factory worker, who appointed himself Bundesführer of the German-American Bund in 1936.

Within three years, the Bund claimed to have 200,000 supporters , although the US Holocaust Museum estimates membership at 25,000. It published a newspaper and had its own SS.

A youth wing staged summer camps such as Camp Hindenberg in Wisconsin, Camp Will and Might in New Jersey, and Camp Siegfried in Long Island, which came complete with an “Adolf Hitler Strasse”.

Jewish groups often protested at Bund meetings, while stronger tactics were deployed by Jewish crime figures such as Meyer Lansky.

Arnie Bernstein, author of Swastika Nation, recounts that one Jewish gangster who sparred with the Bundists was Jack Ruby, then known as Jacob Rubenstein, who would go on to kill John F. Kennedy’s assassin Lee Harvey Oswald.

The commission that investigated the President’s assassination detailed how Ruby “frequently attempted to disrupt rallies of the German-American Bund.” One acquaintance reported that Ruby was responsible for “cracking a few heads” of Bund members.”

Madison Square Garden was undoubtedly the Bund’s high water-mark. Kuhn’s heavily accented address was the centrepiece of the evening. He denounced the “Jewish-controlled press,” which had supposedly cast him as a devil-like “creature with horns” and called for the government to be “returned to the American people who founded it”.

Kuhn did not disguise what this meant in practice: a “socially just, white, Gentile-ruled United States”.

But just as Kuhn was laying into the “Jewish, Moscow-directed” trade unions, a figure rushed towards him. Isadore Greenbaum, a 26-year-old plumber’s assistant from Brooklyn, was tackled to the floor by Kuhn’s thugs and savagely beaten as the crowd booed and jeered.

Badly bruised, he was pulled from the Bund’s clutches by police and arrested for disorder. But Greenbaum showed no remorse when he appeared later than night before a judge.

“Don’t you realise that innocent people might have been killed,” he was asked. He retorted: “Don’t you realise that plenty of Jewish people might be killed with their persecution up there?”

Greenbaum’s brave act was by no means the only protest that night. Outside the arena, a huge crowd of 100,000 demonstrators were kept at bay by 1,700 police — the largest police mobilisation in the city’s history.

They had been ordered there by New York’s legendary mayor, Fiorello La Guardia, who had a Jewish mother and was revolted by the Nazis.

He addressed a 3,500-strong protest rally at Carnegie Hall, but refused requests from Jewish organisations not to let the Bund event go ahead. Even Nazis, La Guardia believed, had the right to free speech.

Madison Square Garden, however, marked a turning point in the fates of both the Bund and Kuhn. The beating of Greenbaum and the subsequent publicity spurred calls for action.

Both LaGuardia and district attorney Thomas Dewey — “Jewey”, as the Bundesfuhrer termed him — were determined to exact revenge.

Months later, Dewey brought charges against Kuhn for embezzling $14,000 from the Bund. While serving his sentence, Kuhn was stripped of his citizenship and, when the war ended, deported to spend his few remaining years in the ruins of the 1,000 year Reich.

But Kuhn’s legacy may not have been altogether eradicated by his imprisonment and fall. Bradley Hart suggests in Hitler’s American Friends: The Third Reich’s Supporters in the United States that thousands of children took part in the Bund’s summer camps.

“We really don’t know much about what happened to them later and what kind of impact the camps might have had on them,” he says.

Nor is it clear what happened to their Bund-supporting parents. Some may well have revised their views with the Nazis’ defeat, but others may not.

“The US government was still asking about Bund membership in background checks for years after the war, so there was still clearly concern about its activities,” Hart says. “For many decades, Americans were living next door to former Bund members and the former supporters of other pro-Nazi groups, probably unknowingly.”

Over the decades, memories of the events of February 1939 have faded. They were, however, reignited when America was confronted by the sight of neo-Nazis and white supremacists proudly parading through Charlottesville 18 months ago.

A short documentary, “A Night At The Garden”, showing footage of the Bund rally, was produced by the director Marshall Curry. Nominated in this month’s Oscars, it is available on to watch online.

But whereas in 1939 America had a president prepared to take his country to war to defeat Nazism, today it has one who describes their modern-day heirs as “very fine people”.