Lincoln did a lot of things that were not legal in the day. He was in a civil war, the "Illegal and immoral war of Northern Aggression against America." This is a man that usurped Maryland's right to secede as that would have surrounded DC with Confederate states. Maryland secession was squelched early in the war and the state had an armed presence throughout the war, if I remember correctly.
My question, however, is any of this authorized by the Constitution? Would it be authorized if Congress declared war?
I think the question is were the debts that were promised to be repayed in Article 6 repayed at the time of the states secession? If not they were violating the compact agreed upon.