It is Constitutional to suspend Habeus Corpus during a rebellion or insurrection. The question regarding Lincoln is whether he had the authority to do so while Congress was out of session. Given the unique nature of the emergency and the stakes involved, I think his actions can be regretted but clearly justified. States don't have the Constitutional authority to leave the Union. If they want to try, fine, but don't pretend it's Constitutional or should just be allowed. That's not a functional nation, but a weaker version of the Articles of Confederation.
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