The Southern States, upon the election of Abraham Lincoln in 1860, who would then take office March 4, 1861, took it upon themselves, to setup within each of their respective States, a special convention, of which such a convention through their State legislature, would vote upon the petition as to whether to succeed from the Union, and thereby having this petition passed, declared that each, in turn, was now a separate and independent State, and no longer part of the Union. This so indicated, that if, for instance, South Carolina's position was legally correct, than any State, at any time, could through a special convention of their legislature, take upon themselves the question as to voting upon their secession, and if passed, simply to leave the Union.
Though the South believed it had the right to succeed, this right to succeed, had previously been addressed in the manner of whether a State could nullify a federal government law by President Andrew Jackson in 1833, in which, that State, any State, if such was permissible, would thereby have the right to nullify any such Federal law of its choosing within its borders. President Jackson saw this nullification for what it really was, rebellion, under the guise of sovereign State rights, which no longer existed, as soon as each State became united into one supreme body politic, and Jackson was prepared to use federal troops to put down such a rebellion.
The Southern States, that than insisted that they had the right to nullify, or to secede, did so, under the false interpretation of the Constitution, and deliberately utilized the term to secede within their respective legislatures, so that they would take on the aura, of legally separating themselves from the Union that since they had previously voted to be part of, they could then at a later date, take a vote as to voluntarily leave from, and if passed, to secede.
The truth of the matter was the Southern States never seceded, for they had not the legal right to do so. When defeated at the Presidential ballot box of 1860, of which, Lincoln won exactly zero Southern electoral votes, yet Lincoln still legally won the election for the United States by his delegate count exceeding 152 delegates with 180, the Southern States thereupon made a purposeful determination that if they could not win the Executive office, which they did not, that they had the right to secede from such.
Lincoln, quite properly, saw the Southern secession not as secession, but as an active rebellion, in which the Southern States by firing the first shots of what would become the Civil War at the Federal soldiers inside Fort Sumter, made it a foregone conclusion that the Union would put down such a rebellion, and at no point did Lincoln ever consider, any other possible recourse, but to take up arms to defend the National Union, and to defend federal property and its outposts, and to defend the Constitution, which he as President, had been sworn to "preserve, protect, and defend."
All this, Lincoln did, for this is a nation of fifty States, indissoluble, secession is not a legal possibility for any State, and those that think so and implement such, are traitors to the national government, and thereby are in active rebellion.