Archives from Stalin’s secret police have become a flash point because of the rise of a movement that has sought to idolize Stalin as a leader who defeated Nazi Germany, spurred industrialization and made the Soviet Union a superpower. Last year, the Kremlin promoted a study guide for high school teachers that deems Stalin “one of the most successful leaders of the U.S.S.R.,” while describing his “cruel exploitation” of the population. Mr. Putin himself has acknowledged the losses under Stalin, but has said Russians should not be made to feel ashamed of them. “We do have bleak chapters in our...