I am not basing the conclusions of the SCOTUS on the Articles of Confederation.
The SCOTUS based their decision on the fact that the US Constitution strengthened the idea established by the Atricles (i e., it is unconstitutional and illegal for a State to leave the Union).
States can consider themselves to have left the Union (as the Confederate States did). But the United States will not acknowledge that States (they will always be a part of the Union as long as the US exists as a nation).
Texas could try to leave the Union illegally, but to actually leave they would have to defeat the United States.
The Articles of Confederation is the only place you can claim 'perpetual union'. And, as I said, they were tossed out. There went that perpetual union with them.
No, the Yankee Federal govt. first of all considered the Southern States to have left the Union. Again, they would not let them back into the Union until they voted for the 13 and 14 amendments. Plus, because the Southern States didn't agree to vote for the 14 amendment, they were then divided up into military districts and placed under martial law for about 12 years.
It wasn't illegal to leave the union in 1861. After military defeat and take over, after creating a Supreme Court in the likeness of the victors image, the Yankees then changed the Constitution that supported the South. And that is the Supreme Court of 1869. The Southern states are out of the Union.
Of course the U.S. won't let us go. It is established by the bayonet. It will be kept by the bayonet. "Mine eyes have seen the glory...."
"When in the course of human events it becomes necessary for one people to disband the political bands which have connected them...." See? America doesn't believe that anymore unless some secession is taking place elsewhere in the world. Then we go out of our way to support it.
Quantrill