It has been argued that the German SS, under Hans Kammler's special technologies office, was experimenting on a craft of unknown specifications beyond a vague notion that it looked a lot like a "bell". It was in fact called "Der Glocke". This was in 1944.
Over twenty years later, an object similar to Der Glocke did an airshow above Ohio, Indiana and finally crashed in Pennsylvania (near Kecksburg). The American military and security services shut that town like it was Alcatraz in the bad old days.
Personal: I was 16 in 1966, and remember the airshow. I had relatives in Pennsylvania who remembered the other stuff.
Now: If the Germans indeed made a deal with the devil, or the Tibetans, or stole technology from Ancient Sources or modern ones (Tesla is suspected) is it possible they did all this? And if they did all this, did the US government know enough to be waiting for them in 1966 to make a pinch?
Or is this about something else entirely?
Below is a link from one chapter of a book which deals exclusively with Kecksburg and what it all might mean. If German technology of the sort Nick Cook was investigating in The Hunt for Zero Point was as out-of-bounds as some of us think, the idea that Hitler & company was in league with an alien power is not so odd as it might first appear.
But read on before you make any conclusions. There's high strangeness in World War II, and this is one of the areas no one dares address. (That does NOT mean it ain't fun to read though!)