The South already left once before. They lost, in part, due to lack of resources (Manpower, Money, etc).
Texas alone would face the same fate as it did the Last time. The addition of California makes this plausible.
That said, I don't see two states more opposite of each other in how they approach things making this very implausible.
It's more plausible that the makers of the movie didn't want the movie to be extinct on arrival by isolating a large part of the movie watcher because of how current politics in the Americas are working today. The movie already probably is skirting on that line.
I think it's going to be so close to that line that it's going to be hard to discuss the movie overall when it comes out it may be best just to avoid it at that point (at least in forum discussions).
PS: The REASONS given for the War is that the President refused to give up power and took a third term. Not going to extrapolate, but just thinking on that can give many reasons why this will be a controversial movie in discussions.