‘Operation Mistletoe’ was purportedly cooked up by James Bond creator Ian Fleming during his time in Naval Intelligence, and was intended to use Crowley’s occult powers to lure the Deputy Führer to Britain. The ritual, held in Ashdown Forest, involved a large number of soldiers dressed in ad hoc magical robes, and either a burning dummy in Nazi uniform or a symbolic model aeroplane which flew down on a cable stretched from a church tower to a nearby tree, accompanied by considerable pyrotechnics and much ritual chanting. (In some versions of the tale, two German SS officers, codenamed ‘Kestrel’ and ‘Sea Eagle’, had been somehow duped into attending the Ashdown Forest ritual and reported back to Hess that the Order of the Golden Dawn was alive and well and waiting to take power once peace was established).
Cecil Williamson (a former intelligence officer and subsequently the first owner of the Museum of Witchcraft) also describes this ritual in what appears to be confirmatory detail, but in previous correspondence with Gerald Yorke (one of Crowley’s literary compilers and later the Dalai Lama’s emissary to Britain) Williamson remarks that he’d never met Crowley. Therefore, if Williamson was at Ashdown Forest then Crowleycouldn’t have been; and if Williamson was not there, how can he claim to give a firsthand account?  The most parsimonious answer appears to be that Williamson is regurgitating Amado’s tale as if it were his own. More tellingly, there is not a single reference to any such event in Crowley’s own voluminous magical diaries and no independent account has ever arisen; nor can any record of soldiers being deployed be found in the National Records Office or Military archives, while the physical geography described in the tale does not fit current or past maps of the area. All in all, it’s extremely unlikely that anything like the event described ever happened, in Ashdown Forest, or anywhere else. 
What does appear to be true is that Ian Fleming suggested Crowley be used to question Hess about Nazi occultism following his capture. Crowley was keen to help, writing a letter to the Director of Naval Intelligence in 1941 stating that: “[I]f it is true that Herr Hess is much influenced by astrology and Magick, my services might be of use to the Department in case he should not be willing to do what you wish.”  Crowley was ultimately not employed, his reputation (and his pro-German statements during WWI) perhaps preceding him.