

The civil war only happens if the U.S. attacks after a secession, but the state that single most funds the U.S. military is California, if you include Washington and Oregon in the secession you now end up with 19ish% of the countries budget and a sizable percentage of the military bases. With the American civil war there was somewhat defined lines of north vs south but this would be fights inside their own states.
You need to amend the constitution which requires 38 states to agree to it that considering how many right wing states hate California doesn’t sound impossible