This past fall, the French podcaster Philippe Collin traveled with his millions of listeners to an enormous, almost 900-year-old castle in southwestern Germany, where members of the Nazi-backed French collaborationist government retreated after the D-Day landings. To the sound of laughter and clinking champagne glasses, Mr. Collin’s listeners heard how the French collaborators celebrated Christmas in the castle’s grand mirrored gallery during the last year of World War II and planned their return to Paris on the backs of German tanks. It’s a part of French history that is little known, decidedly inglorious and — to many in France —...