Secession is “illegal”? Please tell me what law it breaks? Why were never any Confederate officials tried for secession? What part of the Constitution told the states the union would be permanent so they could know before they ratified it?
If any state can leave any time it feels like, what is the point of the union? There can be no such thing if every state is supposedly sovereign. The idea of any binding federal constitution is likewise rendered superfluous. In short, there can be no United States of America because there would be no United States.
But all the southern states agreed to sign the constitution therefore binding them to a united country. It would be no different than dissatisfed people in my state of Wisconsin deciding to secede and firing on federal institutions similar to the firing on Fort Sumter. I'd favor the federal government to apprehend and hang those people. There are no forms for legal secession as set forth by the constitution. The only way to separate is with an illegal rebellion. Like what the southern states did.