The belief probably stems from the Nazis’ forays in the occult (giving birth to the “Nazi mystique”) and the reports of foo fighters by Allied warplane pilots during WWII.
The Luftwaffe were the first military flying branch to utilize rocket and jet propelled airplanes and had numerous advanced prototypes by the end of the war. The design of some of these planes,
like the Horten Ho 229 flying-wing,
were radical for the time, and it’s not hard to imagine the reactions of young US and Russian soldiers who came across prototypes and plans for these bizarre looking aircraft. There is also the fact that Nazi Germany was the cradle of modern (space) rocket technology and that the people responsible for it later became very visibly involved in the US space effort. Robert Heinlein’s Rocket Ship Galileo (in which the Nazis have gotten to the Moon first and established a base there) probably also bears some responsibility
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