I understand what the Supreme Court said. There are a few problems with it though.
1. The founders wholly rejected that notion. The Declaration of Independence says
>When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation
>That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness
>But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.
The Colonies were allowed to rid themselves of the British Government but the states aren't allowed to rid themselves of the American government? That makes no sense. The Founding Fathers would have had no issue with secession. Either the American Revolution was illegitimate or the southern one was legitimate. The British Empire made the same argument that the Supreme Court did. The British were wrong just like the Court.
2. If a state cannot leave, then how did the Southern States become military districts? Where in the Constitution does it give the federal government the right to strip states of statehood?
3. There was consent of the states who left to leave. I'm not sure why another state needs to be involved since the 10th amendment doesn't require the consent of other states.
4. If you have a glass of water. You take a sip of water you still have a glass of water. If a state leaves the Union you still have a union. If changing the amount of state destroys the union then I am not sure how adding to the Union does not do the same.
The Supreme Court ruled this way to provide cover for the unconstitutional actions of the federal government. They banned actions that were both allowed and they themselves did. To use a word from the Deceleration of Independence, that is despotism.