In the aftermath of last week's massacre of 50 Muslims at a mosque in Christchurch, New Zealand, it has become common to equate white nationalism with radical Islam. A typical comment came from U.S. Sen. Elizabeth Warren: "In the same way that ISIS and al-Qaida terrorism pose a threat to the U.S.," she said, "so does the rise of white nationalism."

This perspective is understandable. Right-wing extremist violence is a major domestic threat. According to the Anti-Defamation League's database, it has accounted for about 73 percent of terrorist-related murders in the U.S. in the last 10 years.

It's also true, as Warren also suggested, that U.S. President Donald Trump has failed to forthrightly condemn white nationalists, such as after the deadly violence at a white-nationalist rally in Charlottesville, Virginia, in 2017. In recent years, the Department of Homeland Security has even cut funding for some programs focused on countering white supremacism.