[FPSchazly]

It can be whatever you want! You could start with all out war from the beginning, but I like the campaigns where it's a slow burn and the conflict starts about 1/3 to half way in. Personally, I'd probably make a campaign for a limited conflict, it's hard to say WW3 could happen without nuclear annihilation (or at least go on for years without nukes).

"The difference between fiction and reality? Fiction has to make sense." -Tom Clancy.

But, I would also say that good gameplay does not necessarily need a good story haha the story can just be a kick-off point for some fun scenarios.