The vehicle attack that left a civil rights activist dead and 19 others injured on Saturday was the product of continuous incitement to violence across extreme rightwing echo chambers. After police announced the disbanding of the far-right protest in Charlottesville, Virginia, the live stream chat room of the “alt-right” writer Baked Alaska was flooded with calls to kill Jews, black people and counter-protesters. And members of the anti-communist channel in the chat application Discord vowed to push back harder “until the enemy is completely defeated”.

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Shortly after the incident one user of the message board 4Chan wrote of the killer: “Whoever he is, he is a hero. I salute him.” Others expressed regret that the car had not been bigger, reflecting a climate of denial, justification and belittlement around the vehicle attack – mirrored in the White House’s muted response.

With thousands marching under the banner of “You will not replace us”, the event represented one of the largest gatherings of the US extreme right this century. Billed as a protest against the removal of the statue of the confederate general Robert E Lee, and in defence of freedom of speech, the rally attracted a broad church of groups from libertarians to white supremacists. Key figures from the alt-right, the far-right movement in the US, rubbed shoulders with militant constitutionalists, southern nationalists and neo-Nazis. Several branches of the Ku Klux Klan were in attendance, as well as delegations representing the white-supremacist Daily Stormer website and the notorious National Socialist Movement. The event was endorsed on the other side of the Atlantic by the Greek ultra-nationalists Golden Dawn and self-described German identitarians.

By hijacking topics such as the preservation of southern heritage, free speech, anti-left sentiments, and pro-white and anti-immigration attitudes, the organisers gained traction across this broad spectrum of extreme rightwing thinking. While the event was framed on /pol/ – 4chan’s “politically incorrect” message board – as a way to fight “a totalitarian communist crackdown” and “defend the right of southerners”, the Daily Stormer was rather more frank in promoting it as the starting point “to end Jewish influence in America”.

Quick guide What is the 'alt-right'? Show Hide Who coined the term 'alt-right'? The white supremacist Richard Spencer devised the term in 2010. He has described the movement as "identity politics for white Americans and for Europeans around the world". What does it stand for? The movement supports extreme rightwing ideologies, including white nationalism – used interchangeably with white supremacism – and antisemitism. It positions itself broadly against egalitarianism, democracy, universalism and multiculturalism. Some "alt-right" supporters have argued that their hardline, extremist positions are not truly meant, but are a way to disrupt conventional and accepted thinking. Memes, irony and ambiguity are sometimes used in an attempt to wrongfoot critics. How does the 'alt-right' relate to the Trump administration? The Trump administration includes figures who are associated with the "alt-right", including the former Breitbart News executive chairman Steve Bannon, who worked as chief strategist in Trump's White House in 2017. Many of Trump's policy positions have won favour with the movement.

This illustrates a dangerous convergence of ideology and goals from groups that have traditionally been fragmented and prone to internal conflict. “Infighting is part of every movement – but it doesn’t have to be,” the radical libertarian politician Augustus Invictus, who was announced as a speaker at the rally, posted on Facebook. A few days before the Charlottesville rally, an article explaining Why We Should UniteTheRight was circulated on Gab, the alt-right’s Twitter equivalent. “We’re in the earliest stages of a mass movement” wrote the alt-right blogger Hunter Wallace.

This coalition-building includes overtures to (relatively) less extreme groups, commonly labelled the “alt-light”. For example, several members of the Fraternal Order of Alt-Knights, the Proud Boys and reporters from media such as Infowars and Rebel Media expressed their sympathies with the marchers. Although spats still occur within the far-right ecosystems, this attempt to cross ideologies and borders to unite far-right factions represents a global trend: across Europe identitarian movements are strengthening ties, while groups such as Britain First and Scottish Dawn are collaborating with the Polish extreme right. By focusing on common ideological threads, and putting aside differences, the extreme right is actively trying to mobilise en masse.

Radicalising moderates is a key objective for groups at the heart of this movement. “We want to hit the average. We want normal people,” Andrew Anglin wrote in the Daily Stormer. Tactics vary, with several groups privileging the weaponisation of internet culture to reach the young. Some obsess over aesthetics in an apparent attempt to distance themselves from neo-Nazi insignia and imagery, to present a more approachable face to outsiders: the Ku Klux Klan urged members to dress casually, while for Anglin the priority is “to be hip” and sexy”. The rally certainly acted as a powerful publicity boost, with the Discord server increasing its membership by a thousand in the week leading up to it.

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Our analysis at the Institute for Strategic Dialogue (ISD) suggests that attendance was motivated by a variety of grievances and ideological triggers. Examining Twitter traffic around the #UniteTheRight hashtag, we found that 31% of expressed grievances focused on race – with keywords such as “white genocide” and “anti-white” – and 27% on the left, with frequent denouncing of anti-fascists and communists. Then came frustration around freedom of speech (22%), southern heritage (13%) and the establishment (4%). However, all converged in the concern that the heritage, privilege and future of the white man is under threat. Let there be no doubt that this a white supremacist phenomenon.

The fact that the extreme right has been able to mobilise people from across its hitherto disparate ideological spectrum and galvanise thousands into promoting and attending should be a wake-up call. And this has happened as the Trump administration ends US government support for countering far-right extremism in order to focus exclusively on Islamist terrorism. All this should signal the need to take the danger posed by neo-Nazism and white supremacism – cloaked in broadly political, non-violent rhetoric – more seriously.

At ISD we will continue to closely follow the trends and trajectories of extreme rightwing groups and to push back against the normalisation of hateful ideologies. But this is incumbent upon all of us: we must all be vigilant and must all mobilise to prevent the polarisation that enables the rise of far-right extremism – which spawns terrorist attacks of the sort we saw on Saturday.

• Julie Ebner and Jacob Davey are researchers at the Institute for Strategic Dialogue; Ebner’s book The Rage: The Vicious Circle of Islamist and Far-Right Extremism is published by IB Tauris next month