“Dear Henri, we are in good health in a railway truck that is probably taking us to Holland. We are thinking of you very much, we are sorry to be going so far away,” 13-year-old Blanche Zybert wrote in a scrawled letter, dated September 21st, 1943. The letter, along with instructions and a plea to post it, was thrown from one of two trains that had left a repurposed military barracks in northern Belgium the day before, together carrying 1,434 Jews to Auschwitz-Birkenau. Only 51 would survive the concentration camps. More than 25,000 Jews, and several hundred Roma, would pass...