The above scene from The Rocketeer (1991) posits rocket-pack powered Nazi of the Third Reich taking over the world.
But before The Rocketeer, and even before the whole myriad of "Nazis win World War II" alternate history genre fiction, in 1942, the war wasn't a done-deal.
So LIFE magazine published an article in the March 2nd, 1942, issue that gives six different invasion routes to take over the United States.
It's a pretty interesting read, with Japan invading through Dutch Harbor or invading Hawaii, Germany striking the East Coast, and many of them rely on 5th columnists attacking from within.
It's also an interesting look into early WWII American concerns about the course of the war.
This magazine article would be a great resource for any WWII or Inter War period game.