There'd be a war because of ports, military facilities, oil, and, because, politically California may on average be more slightly liberal than the rest of the US, but overall it looks a lot like the US as a whole, with very liberal coastal enclaves and an ultraconservative inland agricultural heartland, and much of the latter would revolt from California to stay in the US in the event of an attempted Calexit.
There'd be a war because of ports, military facilities, oil, and, because, politically California may on average be more slightly liberal than the rest of the US, but overall it looks a lot like the US as a whole, with very liberal coastal enclaves and an ultraconservative inland agricultural heartland, and much of the latter would revolt from California to stay in the US in the event of an attempted Calexit.