States may think they can secede, but the FedGov would surely differ. That was what the civil war was about, remember? The Republic of Texas, being wary of FedGov when it became a state, affirmed to the FedGov that it had the absolute right to secede at any time. To this day, when the Texas legislature convenes, their first vote is whether to remain a US State, or to secede. They were the only state to memorialize this legally when they joined the union.
If a state like California tried to secede, I’m sure the first thing the FedGov would do would be to economically blackmail them. Cease all Federal payments, certainly, and then demand reimbursement for ALL federally funded infrastructure within the state. Can you see a bill for a $ Trillion reimbursement for the US Highways within the state…. and the list goes on. The state would be demanded to ‘buy out’ of all the federal investment ever done in the state. Then would come the shooting war… and we know who has the biggest guns.
It would indeed be a tragic day for the country.