'Antifa', short for anti-fascists, have become a lightning rod in Donald Trump's America.
They are the often masked protesters seen clashing with the far right on streets across the country.
The Antifa movement has attracted more and more attention in the last year, perhaps most memorably shutting down a speech by right-wing provocateur Milo Yiannopoulos in Berkeley.
And then for its presence in Charlottesville, Virginia, where one person was killed and many injured when a white supremacist drove his car into a crowd protesting against a planned 'unite the right' event.
Anti-fascism is of course not a new idea, but the modern version of this radical resistance has been electrified by Mr Trump's victory.
Many Antifa members blame the President for a measurable rise in hate crimes and far right activity since his election.
To some, anti-fascists have become revolutionary warriors for justice and a crucial last line of defence against the forces of racism and bigotry that can seem all too close to the surface here.
But to others, including some in the police force and to an extent Mr Trump himself, who called them "the alt-left", they are dangerous criminals hell bent on burning it all down.
We went to Portland, a city with a long history of both white supremacist and anti-fascist activity, to find out more.
Many of those who spoke to us insisted on a degree of anonymity, and so most of the names below are nicknames or pseudonyms.
Our introduction to this secretive world of activism began at night.
We are invited to film as a group flyers a neighbourhood with posters declaring that a local resident is a member of the American Front, a violent racist gang.
Direct Action Alliance member Jacob tells us that some other activists have done the "necessary reconnaissance" to know their target's dog walking route, and where he buys his milk.
A team of about 10 hooded and masked members work at a rapid pace, stapling the flyers and 'fight racism' posters to lampposts and gluing them to the pavement.
Jacob said: "What Antifa is is basically community defence, what we do is make sure the people in our community are safe.
"That's what this is, is letting the community know that there's a dangerous person who lives with them and so they can take the proper precautions to protect themselves."
He explains the anti-fascist movement has been busy since Mr Trump was elected.
"The Trump presidency has really wakened a lot of folks to realise… white supremacy isn't over, racism isn't over.
"It's alive, it's prevalent and it's well, they've just been hiding."
All of a sudden, the group gets spooked.
They've received a warning that the white supremacist and his friends know we are here.
One says: "You guys gotta go", and the group melts away into the night.
The next day, a coalition of Antifa protesters allow us to film as they disrupt an anti-immigrant rally held by a far-right organisation called Patriot Prayer.
Patriot Prayer says it is not a white supremacist group, but those who oppose it say it provides both a platform and cover for hardcore racists.
It grows tense as the two sides stand nose to nose. One Patriot Prayer supporter tells a Latino woman to "go home". She pushes him away.
One protester tells me: "We can't give fascists an inch or they will take a mile, we've seen that before in Europe, they have to be quashed when they are small and by any means necessary."
"Any means necessary" can mean physical confrontation, which is exactly what happens.
The fist fights continue for long enough for people to be injured on both sides.
The hatred, fear and anger on display is shocking to watch. Patriot Prayer moves off towards the river, surrounded by the police, as Antifa protesters track them down the street.
There is little love lost between law enforcement and Antifa, who reject the police as a racist organisation.
The police in turn often view them as troublemakers, or even criminals, who prioritise their own principles at the expense of the law.
As we are moving, an Antifa member called Isaiah explains the dark dress code and masks to me: "The tactic is known as black bloc and it is a tactic that we use to make it harder to be arrested by the police.
"In its basic sense, it’s harder to differentiate between two of us.
"We (also) cover our face because many people on the far right dox us; they take our photos, they post it online, they find our home addresses and cell phone numbers, they send us death threats, they attempt physical violence against us, if they don’t know who we are we're safe, essentially, it is self-protection."
I ask him what he says to the accusation from the far right that doxing, or the releasing of personal information online, is exactly what Antifa does to its targets.
He makes no apology for it: "Quite frankly, neo-Nazis believe that at some point in time they will have to commit horrific acts of violence in order to see their world view achieved, to murder non-white en masse.
"If they hold those beliefs in my opinion they should be exposed for it, they should be publicly shamed for it… Yes, I find that perfectly acceptable."
Rose City Antifa member 'Lucy' tells me that much of their group's most important work happens away from the streets, often using sophisticated techniques to track down white supremacists online.
"I think they get much more scared when we know their names, when we know their addresses, where they work, that certainly makes them more afraid.
"When we release someone's place of work, our hope is to get them fired.
"Any second that a Nazi is spending looking for work, or looking for housing if they've been evicted, is time they're not spending organising."
Lucy's colleague 'Carter' says: "It's a very genuine threat, you know they're organising, they're emboldened, and they are committing murders.
"Dylan Roof, a neo-Nazi, murdered people in South Carolina… There have been cases in Portland of anti-fascists being attacked and neo-Nazis consistently murder people here and around the globe.
"These are real threats to our community, and the more we oppose them, the safer our communities are."
In Portland, Jeremy Christian has come to embody that threat.
Last May he stabbed two men to death when they tried to stop him shouting abuse at some schoolgirls who were travelling on the train.
Destinee Mangum was one of them.
She told me: "He said we should go back to where we came from and we shouldn't be here and we don’t deserve to live.
“His eyes were kind of black almost. and like I was just staring at him while he was talking and he was looking right back at me and that's when I was terrified."
Historian Mark Bray argues that when there is such an acute, rising threat from the far right, radical politics is necessary, however disruptive.
He said: "How many bodies have to pile up before we decide to respond to this group that has demonstrated its genocidal intent historically and is targeting people today?
"There's that question of immediate self-defence, but then there's also that question of what I call 'pre-emptive self-defence', which is that do you assume that far fight violence is inevitable? Or do you assume that if you leave these groups alone they may remain peaceful?"
Anti-fascists don’t give them the benefit of the doubt, they say it is a matter of when, not if they will be violent, and in that sense confronting them physically and shutting down their events is part of their repertoire.
He said: "Most anti-fascists are revolutionaries who want to create a new world and in their view part of doing that means breaking with the rules that are meant to keep this world, to keep this capitalist society how it is.
"So in that sense they are not bound to questions of legality, they are bound to questions of ethics and political values."
It is this willingness to break the rules in pursuit of their goals, to use "any means necessary", that has prompted some to argue that Antifa should be classified as gang members or even terrorists.
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But they care little for the controversy.
For most of them, the act of resistance is both a moral obligation and a means of survival.