British military historian M. R. D. Foot has confirmed the degree of German penetration of SOE's Dutch networks, something SOE denied during the War. In his history of the Special Operations Executive, it was confirmed the Allied intelligence effort in the Netherlands had been penetrated from the start of the war. Two Secret Intelligence Service (SIS) agents, Captain Sigismund Payne Best and Major Richard Stevens, had been captured in November 1939 during the Venlo Incident. This culminated in the capture of some 50 British and Dutch agents by the and the during their Operation North Pole.