On August 6, 1945, Toshiko Tanaka was a six-year-old on her way to school in Hiroshima, Japan, when, at 8:15 a.m., she looked up and watched the sky overhead turn blinding white. Tanaka didn’t talk about what happened next for more than 60 years: the burns that rendered her unrecognizable to her own mother, the corpses on the city’s riverbanks, the illnesses that struck down seemingly uninjured survivors, and the once-unimaginable devastation made real after the US dropped an atomic bomb on Hiroshima—the first of two times nuclear weapons have been used in conflict—near the close of World War II.…