John Scalzi writes here explaining the self evident truth that Inglorious Basterds is not a science fiction movie. Of course, a sensible thesis like this is greeted by the world at large as a challenge, and like the pantomime characters the internet types (we, mea culpa!) are, back comes an all mighty "Oh yes it is!" from Philip Palmer.
Well, Oh no it isn't!
There are two decent definitions for sci fi - the academicky one about elements of change examined using the tools of cognitive logic within a fictional framework, and the more pragamatic one of "that thing with space ships and time travel and that".
Inglorious Bastards is neither of these, and saying "Aha! The Man in the High Castle! Eh, eh?" isn't going to make it so. The Man in the High Castle does two thing Inglorious Basterds does not do. Firstly, it poses the "What if...?" question, what if the Nazis won. So, we have re-imagined present day world with America divided down the Rockies between the Nazis and Japan, and some thinking around how that world might work.
I'm inclined to rule out alt histories as another sort of speculation, but what separates The Man in the High Castle from - say - Fatherland by Robert Harris is that it explicitly addresses the concept of the alt history. It's not just about the Nazis winning WWII, it's about what we do when we imagine those things. It addresses the fictional form and the metaphyical questions it raises - this is what makes it SF, although even then a particularly rarefied and Phil K Dickian variety.
Norman Spinrad's The Iron Dream is more easily dealt with, as the main narrative is in fact a parody of a secondary world that is very clearly of the SF/fantasy type. The stuff about Hitler as less of an alt history tale than a deliciously toothy satire on the nature of SF and fandom at the time Spinrad was writing.
Why would you want to call Inglorious Basterds a sci fi movie? What particular insight does it bring? How does it help us read and understand the movie, particularly when one considers QT's consumate - perhaps unrivalled - ability to manipulate genre and scare out the truth that often hides in the easy shapes that genre offers creators. Trying to out do him in this regard puts any critic on a hiding to nothing: genre is the tool he's using and he uses with great care and deliberation. To try and second guess him in this regard nullifues the enterprise.