[PatrioticAmerican]

What was "illegal" about their secession?

[Non-Sequitur]

Their act was illegal in that it was in violation of the Constitution. The original states entered into a union with the other states under terms that all had to agree to. Subsequent states entered into the Union only with the agreement of a majority of the Congressmen representing other states. There was no way to leave the union except through rebellion OR with the consent of the other states. Those are Chief Justice Chase's words, not mine. And he wrote the majority opinion in the Supreme Court case which found arbitrary secession illegal.