I think it’s quite possible to watch a ton of WWII-related movies – both dramas and documentaries – without ever running into the term “theatre” (in its specialized military sense). Another word in the same category would be “operational” (again, in its specialized military sense). Both terms are, I would guess, used routinely staff officers and by military historians, but not very much by front-line soldiers and sailors (the folks on which many WWII films focus) because the terms deal with technical concepts that aren’t directly relevant to front-line personnel. They’re in a different category from the words “strategy”, “tactics” and “logistics”; although those three words are specialized military jargon too, they’re “transferable concepts” which can be easily applied in various civilian contexts and thus are terms which the general public will commonly have heard. As an example, when General Norman Schwarzkopf, Jr. said “As far as Saddam Hussein being a great military strategist: He is neither a strategist, nor is he schooled in the operational art, nor is he a tactician, nor is he a general, nor is he a soldier. Other than that, he’s a great military man,” I’m pretty sure that most people understood most of the points in that list without any difficulty, but probably had no idea of what “the operational art” referred to.