It would be a war within California. The Central Valley isn't going to willingly leave the U.S. to be ruled by what they see as liberal coastal elites with no counterweight.

And because of that (and the leverage the central government would have because of that), there'll be no peaceful divorce of California from the Union.

The simple solution is to let the eastern part of California stay in the union. The western part suceeds.

Considering the demographics of California, nobody really lives there, so that "war" would be short if such a thing were to occur.

If California does split it'll probably just employ a live-and-let-live policy with their wild eastern frontier. Leave the hillbillies alone and they'll leave you alone.