I have been working around the subject of the Holocaust for more than a decade at the Imperial War Museum. But generally this has been at a distance, researching in archives and institutions. Working on the BBC documentary How the Holocaust Began, released this week ahead of Holocaust Memorial Day, changed that. It took me to forgotten places, sites where historical details are still unknown and mysteries remain. There are still mass killings and mass graves about which we know little. Discovering this, and integrating it with the broader history I know, was a profound experience. There can be a...