It was a mistake to visit Los Alamos in the middle of a presidential campaign that Donald Trump might win. I get that now, after wandering around the New Mexico town that is synonymous with clichés like “cautionary tale” and “Pandora’s box” and “be careful what you wish for.”The shadow over the Manhattan Project — the undeniable feat of scientific brainpower that gave us the nuclear bomb — is apparent nine miles from this haunting town, in a roadside protest sign that quotes Pope Francis speaking five years ago in Hiroshima: “The possessing of nuclear weapons is immoral.” It is...