Parents need to understand how white supremacists prey on teen boys, so they can intervene. Raising teenagers can be terrifying. Our squishy little babies become awkward hormonal creatures who question our authority at every turn. I expected that. What I didn’t predict was that my sons’ adolescence would include being drawn to the kind of online content that right-wing extremists use to recruit so many young men. The first sign was a seemingly innocuous word, used lightheartedly: “triggered.” As my 11- and 14-year-old sons and their friends talked and bantered — phones in hand, as always — in the back...