In 2006, Herwig was working as a reporter for Der Spiegel when he learned—along with the rest of the world—that Günter Grass, the Nobel-Prize winning author, had been a member of Hitler’s S.S. in the Third Reich. Herwig promptly called Grass for an interview. “I wanted to know from Grass, why did you keep stumm for so long, and why did you then out yourself?” he recalls. “Grass told me that, one morning, while he was in the bathroom shaving, he caught himself whistling the tune of an old Hitler Youth song, “Uns're Fahne Flattert uns Voran,” which is the tune of the Hitlerjugend. He said it made him realize how deeply the Third Reich had impressed itself on him, and he decided, as a writer, that his means of trying to come to terms with this would have to be his writing, so that’s what he did.” Shocked that such an admired postwar figure—an icon of conscience—could have concealed such a defining secret for so long, Herwig went to the Berlin branch of the Bundesarchiv, where files of the Nazi era are kept, to see if he would find other familiar names. What he saw in those files, he writes in “Die Flakhelfer,” was “a political-cultural pantheon of the German Postwar era.”

“They were the last people you would have expected to be members of the Nazi Party,” Herwig said. The names included “leftists, Communists after the war, very educated, upright democrats.” Growing up in the Federal Republic of Germany in the 1980s, Herwig had “learned about the Holocaust in high school, learned about the Third Reich, learned even about the crimes of the Wehrmacht [Germany’s army in the Second World War],” he said. “I really thought I lived in an enlightened age, and that Germany had come to terms with and owned up to its Nazi past. It was only when I discovered these files that I realized: Wow. There’s a lot of hidden information here that they didn’t tell us about.” He wanted to know, he said, why the names had remained hidden for so long....