Over the past 18 months, investigators across Europe have dismantled a string of neo-Nazi groups. Some were well organized and armed with guns and knives. Others appeared looser. In Britain, a teenager was arrested and charged with plotting an attack to start a race war. These seemingly unrelated cases shared a thread. In each, the authorities linked key figures to a far-right group known as the Base, which recruits online, largely through white supremacist memes and propaganda. The group’s message is that multiculturalism has made Western society irredeemable. Recruits are urged to commit sabotage and murder to hasten its collapse....