From the Articles of Confederation:
"II. Each state retains its sovereignty, freedom, and independence, and every power, jurisdiction, and right, which is not by this Confederation expressly delegated to the United States, in Congress assembled.
It's not your opinion nor mine, but was to be the guaranteed right of each sovereign state of the Confederation. The SCOTUS opinion against secession was made as such because their opinions were bought by the FedGov. They never really gave proof of its unconstitutionality, they just ruled that it was through conjecture. Now we have a court that interprets law as it wishes without constraints supposedly afforded per Paragraph II of the AoC.
Facts of the Case
In 1851, Congress authorized the transfer of $10 million worth of United States bonds to the state of Texas. The bonds were payable to the state or bearer and were to be redeemable in 1864. In 1862, during the war of rebellion, an insurgent Texas legislature authorized the use of the bonds to purchase war supplies. Four years later, the reconstruction government tried to reclaim the bonds.
Question Presented
Was Texas a state in the union eligible to seek redress in the Supreme Court? Could Texas constitutionally reclaim the bonds?
Conclusion
In a 5-to-3 decision, the Court held that Texas did indeed have the right to bring suit and that individuals such as White had no claim to the bonds in question. The Court held that individual states could not unilaterally secede from the Union and that the acts of the insurgent Texas legislature--even if ratified by a majority of Texans--were "absolutely null." ." Even during the period of rebellion, however, the Court found that Texas continued a state.
This was a precedent-setting decision. It is settled case law. If you do not like the decision, then you need to do as others have done in the past - change the law or amend the Constitution. In a country that respects the rule of law, you can't pick and choose the laws you want to follow, or the decisions you will respect. I personally think Roe v Wade is a crock of crap, and I have worked to elect people who will change (or re-interpret) the law. But until that time, it is a settled matter.
Second point: The term "ex post facto" means "after the fact" or "retroactive." In the Constitution, there are two clauses that address the issue: I.9.3 states that "No Bill of Attainder or ex post facto Law shall be passed" by Congress, and I.10.1 states that "No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility."
Virtually every court ruling or decision is after the fact. Your argument indicates that you misunderstood the meaning and application of the term. The White v Texas ruling was made in 1869, after the war. Since the Constitution is utterly silent on the validity of secession, the court must look back to precedent and the original intent of the Framers. It is clear the Supreme Court, not withstanding who occupies the seats, is the ultimate judicial arbiter of the meaning and intent of the Constitution. The people, of course, the real sovereign power, have several ways to change the law or the Constitution. The South did not take that course prior to unilateral secession.
Third point: Every state either entered into the Constitution compact willingly, or joined the under its laws. (One can argue about Texas and Hawaii, but it not really germane to this discussion.) I have said on other threads that the constitution did not create a new country or government; it changed the "operating agreement" between the partners. There is several "pre-constitutional laws" (i.e Northwest Ordinance) and institutions of government (Post Office, Army, Navy) that pre-date the Constitution. Think of the Constitutional Union like a general partnership. Once you are part of that partnership, unless your partnership agreement has specific language on how you can get out, you are part of it until you negotiate your way out, or concede your assets. Of course, you can always try and change the nature and terms of the partnership, if you have the votes.
The South didn't have the votes or weren't willing to try and change the terms - so they unilaterally left the partnership. In business, you get sued if you do that. With governments, you get shot, killed, and imprisoned.
But that isn't what the Constitution says or what the Supreme Court has ruled. Implied powers reserved to the United States can be identified and one of those it the power of Congress to approve any change in the status of a state.