There's a long standing debate about whether the USA exists at the pleasure of the states, or whether the states exist at the pleasure of the USA. Those in favor of "nullification" before and shortly after the civil war certainly thought the former. However, SCOTUS in that case and others have come down firmly on the side of the latter. Since states exist contractually at the pleasure of the federal government, the government could grant them the right to secede, but if I lived in CA, I wouldn't hold my breath (although it would be hilarious to see Thiel all of a sudden not live in Trump's America). I've often thought that we should have let the Confederacy go at the time. Was the Union really worth saving? KKK is still influencing elections 150 years later.