Thursday was a busy work night for arcade manager Max Kunz. But looking forward to the 83rd annual Halloween parade in Hanover, York County, he briefly stepped outside as the floats rolled down Carlisle Street. What he saw made him feel ill enough to throw up. As a black truck pulled St. Joseph Catholic School’s float, Kunz, 46, caught a glimpse of a familiar gate. Not believing his eyes, he focused on the words above it: “Arbeit Macht Frei.” The German phrase for “Work Makes One Free” was displayed on the entrance gates of Nazi concentration and extermination camps, including...