Last month, diverse voices came together to mark International Holocaust Remembrance Day, a commemoration designated by the United Nations in November 2005, and marks the date of the liberation of the Nazi death camp at Auschwitz-Birkenau. Also last month, the world watched as a man fueled by vile antisemitic conspiracy theories took hostages in a daylong standoff at Congregation Beth Israel in Colleyville, Texas. Almost immediately, across social media in the United States and beyond divisive and hateful posts began. Islamist extremists in the United States, Great Britain and beyond praised the actions and motives of the hostage-taker. Similarly, white...