>>211658237 (OP)Some people see this movie as a deconstruction of the traditional Hollywood historical epic, particularly those about World War II. Such movies often play faster and looser with history than the average viewer realizes, and can be unintentionally insulting in their reductiveness. By intentionally writing an alternate history, and going for fun action over a serious statement, Tarantino disputes whether such movies really work as actual history lessons, or at the very least, mocks how seriously these movies take themselves.
Or, if you take it as straight-forward, the film is formatted In the Style of a World War II propaganda movie made in 1944-45, just as the Normandy landings were happening. In this, Inglourious Basterds is no different than Mrs. Miniver, Casablanca, Desperate Journey, or other war films made while the war was still raging, meaning the real ending of Hitler committing suicide in his bunker hadn't happened yet. So, Basterds could depict its version of the war however it liked.