Mr. Manson was not the end point of the counterculture. If anything, he was a backlash against the civil rights movement and a harbinger of white supremacist race warriors like Dylann Roof, the lunatic fringe of the alt-right.

Mr. Manson was famously inspired by the Beatles song “Helter Skelter,” which, as he understood it, described a race war that he had been prophesying. Like many reactionaries, he saw race in America in apocalyptic terms. He believed that African-Americans would soon rise up and begin to murder white people. Mr. Manson and his followers would be spared; they were going to hide beneath the desert in Death Valley until the war was over, when they would surface from their underground lair and rule over the black population, which, Mr. Manson claimed, would be unable to govern itself.

But when this race war proved too slow in coming, Mr. Manson urged his followers to set it in motion themselves, to “do what blackie didn’t have the energy or the smarts to do — ignite Helter Skelter and bring in Charlie’s kingdom,” as Tex Watson, a member of Mr. Manson’s “family,” recalled. Mr. Manson assumed that the murders of wealthy, white Angelenos would be blamed on African-Americans and the race war would begin.

Joan Didion described them as “senseless killings.” But they were not senseless. They were racist.

Today, this sort of logic is all too familiar to us. The paranoid, racist and apocalyptic ramblings of Mr. Manson are the DNA of the reactionary alt-right. In the days leading up to Dylann Roof’s murder of nine black parishioners at Emanuel African Methodist Episcopal Church in Charleston, S.C., he talked to his friends about a “race war” and later used the same language in interviews with investigators. He was an enthusiastic reader of alt-right websites.

In recent months, the far-right media have become fixated on the idea that left-wing “antifa” activists will spark a new civil war. Gateway Pundit, a far-right website, claimed that “millions of antifa supersoldiers will behead white parents,” and Alex Jones, the conspiracy enthusiast who runs the website Infowars, predicted that the antifa activists would lose such a war.