Looking at the map on the screen. There ain't no way in hell California and Texas is gonna make a full on assault into DC. There are too many military assets in place that are there to prevent such a thing. Like a whole nother level of raw firepower. Continent glassing firepower.
There wasn't exactly a very stable frontline or interior lines going on. The meat of the movie is a journey from New York City to Washington DC where the Federal Forces ostensibly have control of the Northeast. But traveling through that area, it's pretty clear its quite porous.
Also a third frontline was opened up when the Florida Alliance apparently engaged Federal forces in the Carolinas.
Plus there's large numbers of communities that are staying out of it or trying to pretend nothing is going on.