At the beginning of World War II, the Nazi army had trapped the Allies at Dunkirk, France, and made the British retreat back to England. Prime minister Churchill wanted to be sure that if Hitler invaded Britain, there would be resistance fighters trained and ready. In case of invasion, they were expected to sabotage Nazi aircraft, trucks, tanks, and fuel stations. Some of the units were trained in the radio operations and were expected to act as the spies, reporting information back to the British intelligence officers. Essentially, the secret army soldiers had signed up for a suicide mission. If the Nazis invaded, they had sworn to give their all to protect the Great Britain. The secret of the resistance units was fiercely guarded—not generally made public knowledge until the 1990s, nearly 60 years after the end of the war.