Those favoring California's exit from the United States, the so-called "Calxexit," have qualified to collect signatures to put the issue on the 2018 ballot in the Golden State, according to the Sacramento Bee. Let's imagine for a moment they get enough signatures, put it on the ballot, and it passes. Offhand, I see two likely scenarios.
I fantasize California's then-Governor declaring independence. President Trump, harking back to the example of President Lincoln, declaring that action invalid and placing CA under martial law.
Imagine "Reconstruction II" overseen by General Mattis. Trying the governor and others who actively advocated secession as traitors. Internment camps for rebels, former rebels fleeing to Canada and Mexico.
OR, imagine that Trump says "Don't let the doorknob hit you in the backside on your way out." He then turns off the federal tap, evacuates the military and border patrol from their many CA bases, and sets up custom posts and border controls along the new border.
Absent CA's huge liberal vote, the GOP then wins the next four presidential elections. CA opens its borders and becomes a third world country, see the Matt Damon film Elysium for one view of how this might look.
If I weren't so lazy, I'd write novels about these dystopian futures. I offer them to you as ready-made plot outlines, no charge.