This is the way I like at it, ufo's existed in 1939, just as they exist now. Hitler would've been privy to all information coming into Germany. Say a pilot sees something in the sky, words gonna eventually get to the German high command. If a bunch of these reports are making their way to Hitler, then there's no doubt the Germans are gonna investigate these sightings.

Hell, maybe they were in contact with aliens, that's all speculation. What I am sure of though is that the Germans knew there were aliens zipping around the sky. There's zero proof as far as I am aware that the Germans possessed alien tech or were given any help during the war. But I operate under the modus operandi of 'every nations government is aware there are aliens'. Maybe not every nation, but lets be real, there's probably a small army of people in the U.S. government who know aliens exist. I just don't see how their existence can be denied at this point- especially at a high government level.