These days, when you enter the Imperial War Museum in London, they search your bags. “We’ll all have to get used to it,” a security guard told me. “With the world the way it is.” On your way up through the building, an elegantly proportioned former royal hospital, you pass the entrance to the space where the permanent second World War exhibit is housed. Glancing through the door you see a swastika. There is a gallery dedicated to remembering the Holocaust. Sometimes you see people in tears as they’re leaving. Researching in the museum’s archives this week, I came across...