HENRI CARTIER-BRESSON took the epigraph for his landmark collection of photographs from Cardinal de Retz: “There is nothing in this world that doesn’t have a decisive moment.” Cartier-Bresson’s American publisher settled on The Decisive Moment for the book’s title. Whenever the media confront a horrific outcome, the search for the “decisive moment” dominates the coverage. Two very different recent tragedies—the fatal police shooting of a carjack…