More so than the careers of sensationalized figures like David Duke and Richard Spencer, Mr. Friberg’s path shows the broader trends and transformations of latter-day white nationalism in the West. He grew up during the 1990s, when skinhead subculture attracted waves of disgruntled young white men.

Despite its mobilizing power, skinhead subculture rarely led to much beyond petty violence and rabble-rousing, especially in Mr. Friberg’s home country. By 2000, Sweden had an oversized nationalist skinhead scene with a world-leading white-power music industry but no serious anti-immigrant political party in Parliament to show for it. And while Mr. Friberg had entered activism through neo-Nazi skinheadism—shaving his head and doing most of his political networking at concerts—he quickly began cultivating alternatives, striving, like many of his reformist-minded colleagues, to be everything skinheads were not.