When Gerhard Weinberg returned to Germany in 1962, he hadn’t seen his hometown of Hannover for more than twenty years. The buildings on his old street, rebuilt after the relentless bombings of the war, looked brand new. He stopped across the street from his family’s old apartment building. “I burst out laughing,” Weinberg remembered. He thought to himself that British bombers must have had very good aim. “The one building they didn’t hit was the one we had lived in.”

That house was the site of many unhappy memories. In 1934, Weinberg’s father, who had lost his government job on account of his Jewishness, started using the living room to advise desperate emigrants. A couple of years later, when Weinberg’s classmates started beating up Jewish students, home became a place to avoid. “I would get terrible nosebleeds and, on the way home from school, would hide out somewhere until it stopped,” he said. “I didn’t want to upset my mother.”

Weinberg, who is now eighty-seven and retired, spent his career dredging up ugly memories from the remnants of the Second World War. In 1938, as a nine-year-old, he fled Germany to escape Hitler’s regime. In 1962, as a professional historian, he returned to investigate Hitler’s legacy. By that time, he was not only an émigré from Nazi Germany but also Hitler’s editor: Weinberg had discovered the dictator’s second book, a manifesto about race and foreign policy that had been unpublished and was largely unknown. With help from Munich’s Institute of Contemporary History, he made sure it didn’t stay that way.

In 2016, the Institute of Contemporary History will publish Hitler again, issuing the first German edition of “Mein Kampf” since 1945—over objections that the book could offend Jewish groups or stoke the fires of neo-Nazism. In 2013, a government official from the state of Bavaria—which holds the expiring copyright—threatened to file a criminal complaint against new editions; the culture minister declared that “many conversations with Holocaust victims and their families have shown us that any sort of reprint of the disgraceful writings would cause enormous pain.” Bavaria eventually decided to let the institute proceed, but the threat of a lawsuit hangs over future editions.

The new two-thousand-page text, which surrounds Hitler’s prose with side-by-side commentary and more than three thousand footnotes, comes out in early January. Weinberg says it’s about time. ...