One of history’s greatest “what ifs” is the question of what would have happened had the Germans been able to develop nuclear weapons during the Second World War. The Wehrmacht’s effort to do just that, called the Uranverein, or “Uranium Club,” began in 1939 when German Army physicist Kurt Diebner began to research the potential military applications of nuclear fission. By year’s end, the renowned German physicist Werner Heisenberg had expressed his belief that nuclear fission chain reactions, and thus, eventually, nuclear bombs, might be possible, but only if he had access to enough of a singular substance known as...