If California, or Texas, or any other such state wanted out, I see no problem with allowing them to go. Whether our president would agree, I do not know.
Do you believe that the Soviet states should have not been permitted to secede from the Soviet Empire, as well? How about the E.U., should the other countries go to war with the ones that want to withdraw?
The historical fact that the Civil War cost the lives of at least 750,000 soldiers, created hundreds of thousands of widows, millions of children left fatherless, and the ensuing problems we have had with the South since then, is a tip off to me, that they should have been let go. Are you actually glad they were forced to stay, to drain the economy, enforce segregation for 100 years, keep prayer in public schools as long as possible, fight against abortion rights, and vote for Trump (for which I don't blame them)? You must really hate this country, or have been conditioned, like so many have been, into believing that Lincoln was a great man and did the best he could, for the greater good. Have Trump's worst actions as president ever come within 1% of the war bloodshed and ensuing savagery, injustices, lynchings, the KKK, and other heartbreak that happened because Lincoln would not let the Confederacy leave the Union, or work out some kind of peaceful solution? I think not, but you'll never get it, so I don't even know why I bother to ask.