Their acts of secession were NOT illegal. If you can join a Union you can unjoin it. The constitution gives us the freedom to determine our own government and if that means leaving the Union then that is allowed also. The North violated the constitution, they were the treasoneous ones because they badgered the South until war was inevitable.
Actually they were. To quote Chief Justice Chase:
"When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration or revocation, except through revolution or through consent of the States.
Considered therefore as transactions under the Constitution, the ordinance of secession, adopted by the convention and ratified by a majority of the citizens of Texas, and all the acts of her legislature intended to give effect to that ordinance, were absolutely null. They were utterly without operation in law. The obligations of the State, as a member of the Union, and of every citizen of the State, as a citizen of the United States, remained perfect and unimpaired. It certainly follows that the State did not cease to be a State, nor her citizens to be citizens of the Union."
If you can join a Union you can unjoin it.
Without trying to put too fine a line on things, stated did not 'join a Union'. They were admitted, and only with the permission of a majority of the existing states as expressed through a vote in both houses of Congress. I see no problem with leaving through the same manner.
The North violated the constitution, they were the treasoneous ones because they badgered the South until war was inevitable.
Absolute nonsense.