On 27 April, before he burst into a San Diego synagogue and opened fire, killing one worshipper and injuring three more, the gunman said goodbye to the community that radicalised him. “It’s been real dudes,” he posted on the far-right politics board, /pol/, on the image-posting site 8chan. “I’ve only been lurking for a year and a half, yet what I’ve learned here is priceless.” The story was familiar. Six weeks earlier, a 28-year-old had killed 50 people at two mosques in Christchurch, New Zealand. Before starting his attack, he, too, had posted on 8chan’s /pol/ board. “It’s been a...