Transcript for Who are the white nationalists and Antifa?
When you look at charlottesville, especially that Friday night with the tiki torches, I think the country was shocked because those folks looked like people in your neighborhood. They didn't have the robes. They didn't have the tattoos all over their body. In many cases, they looked like somebody that you might know. Reporter: The shocking images, the killing in the street. Just plowed through hundreds of people. Reporter: The violence in Virginia last weekend was just the most recent and most tragic confrontation between white supremacists and a militant resistance. We're seeing a period of unusual, intense polarization among the population and in the political arena. Just get over there where you belong. Reporter: For the past six months we've been crisscrossing the country tracking all this political violence. Embedded with both sides, from their biggest brawls to their secret meetings. Some of what you will hear tonight will be hard to hear and difficult. But its important to understand how all this began. Our story begins here. Knock the crap out of them, would you? Seriously, okay? Just knock the hell, I promise you, I will pay for the legal fees. I promise, I promise. Reporter: Last year the trump campaign inspired many types of people. Among them, Matt Heimbach, a rising star of a white nationalist movement that calls itself the alt-right. President trump has denounced groups like Heimbach's. Racism is evil. Reporter: But Heimbach says the president been an inspiration. He's opened up a door. His movement has opened up a door but it's up to us to take the initiative. Reporter: The number of hate rose last year to near its all-time high, says the southern poverty law center. Now leaders like Heimbach admit to a newfound energy. I think he's a reflection of the excitement that Mr. Trump has engendered in the white supremacist movement. Get out of here. Get out! Out! Reporter: That's Heimbach at a trump rally last March. The guy with the beard and the red hat. Striking, and then shoving this African-American protester, as she's being led away. He later pleaded guilty to disorderly conduct. There wouldn't be any violence at trump rallies if, of course, the far left protesters hadn't gone into the rallies and tried to disrupt them. Reporter: Isn't there something to be said about decency. Clearly you're on that video pushing a woman from the back. You have a right to defend yourself. Reporter: The antagonists of the alt-right are really a loose-knit group of activists, who prefer to be called the antifa movement, short for anti-fascists. Right now, we're in a very dangerous place, we're in a very troubling place. Reporter: Lacey Macauley, a self-described anarchist and self-described anarchist and mild-mannered worker at a nonprofit by day, was one of the few antifa activists willing to go on camera for this report. A lot of people basically have been responding by caring to join the antifascist movement. Antifa is on the hard left. In fact, many would argue parts of them aren't even left. They're anarchists. Reporter: And they're not afraid to play rough. In San Jose up close, I saw it up close. They brutalized trump supporters at random. Throwing eggs at them, beating them bloody, and attacking their cars. There's no doubt that the antifa believe that physical confrontation is necessary to prevent the rise of white supremacy. Reporter: The protests didn't cool down when trump won the election. The man who will be the 45th president of the United States, Donald Trump. Reporter: The alt-right rejoiced. It was at this notorious conference in November that much of the country was introduced to Richard Spencer, the self-styled intellectual who coined the term alt-right. Donald Trump means that the world is changing, something new is coming into the political reality. Reporter: Several eagerly joined in his enthusiastic Nazi salute. Hail trump. Hail our people. Hail our victory. Let's not sugarcoat this. He's a Nazi. Reporter: For a lot of Americans who saw that, it was terrifying. I knew that I was being highly provocative when I said, "Hail trump." I don't think there's anything wrong with saying that. Reporter: To be clear, most trump supporters aren't alt-right demonstrators. Most trump resisters are not antifa. But those mobilized minorities at the extremes can have a big impact. And January 20th, inauguration day, became d-day for both sides. Many of the people "20/20" has been following for months were right in the middle of it. I, Donald John trump, do solemnly swear -- Reporter: Just blocks away, some members an antifa group lead by lacy macauly disrupt j20, swung into action, torching a limousine and scuffling with police. There's a massive undertaking in the district to actually oppose the inauguration of Donald J. Trump. Reporter: Macauly believes hate speech is the equivalent of violence, and can be answered with violence. We've seen her take on armed white supremacists with little more than a bullhorn. So why do these confrontations always get so violent? I mean, yes, you might have your hot-headed 19-year-olds. Antifa who I know are really trying to actually act in self defense. Reporter: But what about the vandalism and harassment here of an inaugural ball guest perpetrated by some of her associates, using so-called "Black bloc" tactics? Everyone wears the same color. Hey, basic black. It's trendy. And also just, you know, wearing the mask, to conceal your identity. Reporter: The world got a glimpse of these tactics in action when some in Macauley's group, clad all in black, smashed the windows of a Starbucks and a bank of America. Breaking a window is a symbolic act. Windows break all the time. Things break all the time. Reporter: So this is your home. If I -- after this interview, if I broke all your windows in a symbolic act, you'd be okay with that? That would be a symbolic act. I mean, if you wanted to protest this interview, you can break the windows. Reporter: And there that day at a command post in Mcpherson square, daryle Lamont Jenkins, a truck driver from Philadelphia who describes himself as the "Intelligence expert" for antifa, keeping tabs on Spencer, Heimbach, and their alt-right ilk. So you're a watchdog? Correct. We basically just report what we see. We go anywhere they go. Reporter: On the other side, Matt Heimbach was there. And Richard Spencer turned up too. We'll expect some protesters, they'll do silly string. Reporter: Then a turning point. Watch this. Spencer was conducting this interview when, wait for it. A man walked up and punches him, right in the face, in the middle of the street. If you're doing something important you're going to be attacked verbally and physically. I'm willing to go through with it. Reporter: The attack went viral instantly. Sparking an internet debate about whether it was okay to punch a Nazi. Was that okay? I think that you saw a lot of people actually very inspired by the fact that the Nazis are not invincible. Of course it's not okay to punch a Nazi any more than it's okay to punch a doctor who performs abortion, if you believe abortion is murder. The same people who insist that they are in the right when they are engaging in violence would be horrified if the roles were reversed. Reporter: As fate would have it, just hours after Spencer was punched, the roles were reversed. On inauguration night in Seattle, an anti-Trump activist was injured by a gunshot through the abdomen. Police! The only individual who's ever been shot at one of these rallies, he was an antifa member. And he was shot by a Maga hat-wearing trump fan. Here's the reality, 74% of the extremist-related killings in this country in the past ten years have been carried out by right wing extremists. Reporter: Given all that, you'd think Spencer and the alt-right might have wanted to dial things back. But instead, he was just getting started. They brought the fight to us. There was a war started. And those sons of bitches started the war. And so we're going to respond to them. Reporter: When we come back, the alt-right is heading to campus. Trying to recruit America's youth and the resistance is there to greet them. And, inside an alt-right conclave. You never know who you'll meet. I'm a Baptist preacher, so I got to maintain a low profile.
This transcript has been automatically generated and may not be 100% accurate.