I believe that was settled 1861-1865. Also in Texas vs White 1869.
https://www.britannica.com/event/Texas-v-White
“Texas v. White, (1869), U.S. Supreme Court case in which it was held that the United States is “an indestructible union” from which no state can secede.”
The decision of Texas vs White was a biased and bent decision as all eight of the SC members were from the North...Pa, Ill, Oh, Cal, Iowa, NH, NY and Ohio. Several of the SCOTUS members were Lincoln cronies, staunch anti Southern beliefs, and used the lawsuit about bonds to justify the invasion of the South. Even three of the SCOTUS members didn’t agree with the decision. The decision of secession has still not been addressed in a court of law correctly and without bias. The decision made a conclusion of the subject of secession without rebuttal as the lawsuit was about financial compensation for bonds. I applaud Harvard (of all places) for a recent diligent study and came to the conclusion that the North was wrong in invading the South. The firing on Ft. Sumter was in response to Lincoln’s dishonesty in dealing with the Southern representatives,