PARIS — By the middle of 1942, a handful of senior officers in the German army and intelligence apparatus worried that their Führer, Adolf Hitler, had gone completely insane.

That may sound, today, like an understatement. But as happens when any populist demagogue takes power, many people embraced him at first, many others were willing to make excuses for him, and still others convinced themselves that they could live with him, at least. Indeed, over the previous decade the vast majority of Germans were persuaded that Hitler understood them, and they understood him—such was the chemistry between the man and his constituents—even if much of the rest of the world found him appalling.

“He is the loudspeaker which magnifies the inaudible whispers of the German soul,” world-renowned Swiss pyschotherapist Carl Jung told an American reporter in 1938.