probably not, depending on which side CA lands. the western US is probably actually pretty well equipped to stave off the east from invasion. there's, grossly oversimplifying, hella mountains (north) and hella desert (south). not only that, but california feeds a lot of the nation.
honestly, the 3 pacific coast states (CA/OR/WA) are probably pretty well equipped to secede and start their own country if they should see fit to do so. massive coastline, tons of food production, hugely powerful economy in cali.
It still depends on the military. CA is certainly an economic powerhouse, but if the bulk of the military doesn't side with CA, I doubt groups like the National Guard (assuming they don't also support the federal gov't) or private militias (which, let's face it, are children with toys compared to the US military) will be enough to repel the US military, mountains and deserts be damned.
