There was a man who covered the whole subject of a states right to secede. Look up the Innaugural address that was given by the man who was elected president in 1860. The issue of secession has never been so thoroughly explored as in that short read.
Ask yourself this: suppose California upon hearing that Donald Trump had just been elected to the office of POTUS says, "we're out". Would that be ok?
Every state that legalized abortion before Roe v. Wade should have been expelled from the Union. California should have been expelled in 1967, New York in 1970, etc. All the states should have seceded in 1973.
So, if California wanted to leave upon the election of Trump, the United States absolutely should not wage a war to stop them!
Way more than OK! Do it! Without those 30 million kooks the US might survive.