I have a crazy theory that they've gone a different route, I could be wrong but I think theatre of war either refers to a new system that allows players to record battles for cinematic value e.c.t..
OR, and this is the thing. What if Battlefield 5 didn't just focus on one war, what if it spanned an entirety of wars from WW1 > WW2 > Vietnam > Present day.
I think this because of the way they mention Battlefield changing forever, theatre(s) of war makes me consider that it will be sort of a view on a war a war that we'll participate in.
The whole WW2 screenshot being to throw people off the scent.
Of course I could be wrong and most likely am, but its a theory none the less.