It is a proven fact that the southern states did not have to leave the Union to preserve Slavery.
It was protected under United States law until December of 1865 with the illegal passage of the 13th Amendment. The very word "Contraband" and the need for such a term regarding the Emancipation Proclamation is proof enough that liberation of slaves under United States law without due "legal" recourse
(an act of Congress by means of amending the Constitution) was Illegal.
Lincoln did not start the war to end slavery, but to "Preserve the Union". Another Illegal act under the Constitution. The occupation of Fort Sumpter was an "Act of War" because it threatened the commerce of the State of South Carolina and Charleston, South Carolina then being and Independent States. No state or group of states has the right to make war against any other state. Such an act abolishes the Constitution of 1787, which was a "Compact of Cooperation" between the then 13 Independent States.