After a nuclear war between USA and Russia (who between them have enough nukes to destroyed the world several times over) I don't think there will be anything left to fight for (if there is anyone left to fight in the first place).
That will take the series into sci-fi and I don't think the franchise is about that. It is more rooted in "realism".
As I know someone will bring it up zombie mode is not the main feature of the last release.