I find it implausible that it would win enough approval for the Californian government to do anything about it.
I’m having some difficulty getting numbers later than early 2018, but I gather that support for it has been dropping and that the ‘Yes California’ organization had, in response to Trump’s depredations, grown to 44,000 members. I think California has soccer teams larger than that.
More to the point, when a country attempts a secession there are a few different steps to be taken, and I don’t think the California has the stones to carry out the step ‘seize the federal military bases within its borders and launch an attack’. I think that even if it got as far as the “submit a formal secession request to the US Government” step, when Congress laughed at it and told it to shut up I think California would slink away with its tail between its legs. Jumping from “please let us leave” to “send the national guard to arrest my pudgy, suit-wearing butt for treason” is a pretty big step for any politician to take.
ETA: Note that I’m saying that Congress slaps them down, not Trump. Trump can make any damn noise he likes, but Congress is where secessions die.
The hard part isn’t getting the sides killing one another - it’s getting it to happen so quickly and widespread that the cops don’t roll in and arrest the entire conflict. In my opinion terrorist actions do not a war make, and I don’t think that the actual warmongers have the numbers or cohesion to do more than that.