The South rightfully asserted their Right of (dis-)association. They politely asked Federal troops to leave Southern property. Federal troops declined and we're fired upon. Lincoln then forced his Generals to attempt an invasion of Virginia. They failed miserably.
Again, the Constitution had no provision for unilateral withdrawal. Which means withdrawal has to be mutually agreed upon by the remaining and leaving parties, or you get a fight.
In fact one could make a case that an amendment is required to remove a State from the union, setting the bar far higher.
An amendment could legally dissolve the Union, if it passed.
That is what should have happened.
That is what should happen now.
We do not need to be a world superpower, and at least three countries should be formed, in my opinion.
It happened in the USSR, and, it was a good thing for the whole world.