In both, there is the apocalyptic ideology that predicts — and promises to hasten — a civilizational conflict that will consume the world. There is theatrical, indiscriminate violence that will supposedly bring about this final battle, but often does little more than grant the killer a brief flash of empowerment and win attention for the cause.

There are self-starter recruits who, gathering in social media’s dark corners, drive their own radicalization. And for these recruits, the official ideology may serve simply as an outlet for existing tendencies toward hatred and violence.

Differences between white nationalists and the Islamic State remain vast. While Islamic State leaders leveraged their followers’ zeal into a short-lived government, the new white nationalism has no formal leadership at all…

The lessons are concerning. It is nearly impossible to eradicate a movement animated by ideas and decentralized social networks. Nor is it easy to prevent attacks when the perpetrators’ ideology makes nearly any target as good as the next, and requires little more training or guidance than opening a web forum.