The aim of terrorism is terror. It’s an easy tautology to overlook, for it appears to carry no information. But terrorism’s aims are political and social, even when its methods are violent.

When a gunman opened fire in a synagogue in Poway, California, near San Diego, on Saturday he killed one person and wounded three others. Those figures are low by mass-shooting standards, but the attack has broader implications. Anywhere, everywhere, Jewish worshippers might fear similar violence at their own temples. The same distress descended upon Muslims after a deadly attack on three mosques in Christchurch, New Zealand last month. These attacks infect the innocent with suspicion. With no way to know where or when more random violence could erupt, hearts skip a beat with every doorway darkened.

In both the California and New Zealand, attacks, the shooters posted notice, motive, and evidence on the anonymous message board 8chan. In New Zealand, Brenton Tarrant published a manifesto linked on the website and livestreamed his attack on Facebook, via a link also posted to 8chan’s /pol/ (“politically incorrect”) board.