Adolf Hitler in 1940. Photo: ullstein bild via Getty Images Struggling to divine Germany’s intentions in the midst of the Sudeten crisis in 1938, the British ambassador in Berlin, Nevile Henderson, put his finger on the fundamental point that had flummoxed conventional intelligence-gathering efforts against the Nazi government. “It is impossible to know anything for certain,” he reported to London, “in a regime where all depends on the will of a single individual whom one does not see.” The terrifying repressions of a total police state made the most innocuous efforts at penetrating the German regime’s secrets arduous and dangerous;...