"I saw Kenneth Strong from military intelligence this evening. ... Strong has a great regard for German efficiency and cannot bring himself to believe that they could have been so stupid, as to send these ... spies ... over here without having schooled them properly and worked out plans by which they could be really effective.", wrote MI 5's Director of Counter-Espionage Guy Liddell on September 8th 1940 in his diary.
To pave the way for "Operation Sea Lion", Hitler's plan to invade the United Kingdom,, about 20 German spies were infiltrated into Great Britain. But their mission ended in abject failure, all spies were caught.
Mainly based on material in the British National Archives German Historian Monika Siedentopf argues that the "Operation Sea Lion"-debacle was not due to German dilettantism, but the result of deliberate sabotage.
Naval captain Herbert Wichmann and some other officers in the Hamburg bureau of the Abwehr, Nazi Germany's Military intelligence agency, were strictly anti-Nazi and determined to thwart Hitler's intentions.