The allies won the war, appropriated the secrets, and 2 years later they were flying these craft. It’s not so far-fetched a theory as having extraterrestrials come across the Galaxy only to crash at Roswell.
Anyone who wants to come up to speed on this hypothesis would find the book “Intercept UFO” by Renato Vesco to be the original & best material to read.
“This is guerrilla advertising for ‘Iron Sky’, a Finnish movie sci-fi comedy scheduled for 2011, where Space Nazi flying saucers from the dark side of the moon invade Earth in 2018.”