Hello hypocritexposer,
The German-UFO or USAF-UFO theories just don't hold water. Despite some claims about UFOs being Nazi weapons, the German engineers never got their disc designs further than the drawing board. They, and the Japanese, as well as the allies, all saw UFOs--which were called "Foo Fighters" by the allies--during the war and everyone thought that they were the other guy's invention. Google the term Foo Fighter to learn more about the Foos (not the rock band).
IMHO, the most convincing argument in favor of a non-terrestrial origin for UFOs are their capabilities as tracked on radar. Google *UFO Washington D.C. July 1952* for a taste. Several UFOs were sighted and tracked over D.C. as they literally flew rings around our fighters and reached velocities of 7,000 mph. If we, or the Russians, had such craft in 1952, why would we have spent trillions of dollars and rubles in the years since building fixed-wing aircraft with far inferior performance capabilities?
Furthermore, as early as 1954, credible persons (for example, law enforcement personnel) began reporting landed UFOs with small humanoid, but not human, figures standing or walking nearby. Some were 3-feet tall, some 7-feet tall and their facial features were quite clearly not human. The variations in appearance, from case to case, suggest that perhaps races of "visitors" from different worlds have made it here.