Fegelein is behind the invasion.That's where he kept running off to, and the source of all his pranks. FEGELEIN! FEGELEIN! FEGELEIN!!!
Hitler himself will return at the end of the film.No Nazi film is complete without Hitler. Cloned him, time-travel, ascended to a higher plane of existence? Space Nazis will find a way.
The movie is an alternate timeline of Nazi ZombiesIn this timeline the zombies were controllable by their living officers as a result the Nazis had completed their moon base before the Allies inevitable crushing of the Third Reich, as the zombies did not cause a breakout
Based on the ending of Iron Sky's credits, any sequel will involve Soviets on Mars.With the possible title of "Rusty Sky" or something like that.
The Nazis are the heroes all along.