But Lincolns legal argument was that the states never left, and could not leave, that they had always remained a part of the Union.
How do you square that circle?
Slavery did not end until AFTER CWI. 1868 IIRC. Learned this last year.
That is true, the States remained part of the Union. He declared them in a state of insurrection . As such, the President and the Congress exercised their Constitutional authority to used the Army and the Navy to suppress insurrection. Lincoln never claimed in 1861 that ending slavery was the objective in prosecuting a war against the Confederacy.