Ruth Cohen, a 94-year-old American Holocaust survivor, returned to the Auschwitz concentration camp in Poland for the first time. She recalled seeing family members before they were separated for the last time at the camp.

That creates risks: the Holocaust didn’t begin with mass murder. The dehumanization of Jews progressed gradually from public exclusion to eventual internment to finally extermination. Millions of regular Germans—and Europeans more broadly—facilitated or silently accepted these actions.