free dixie,sw
Article VI, Clause 2: This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
No state had the authority under the constitution to pass any law or make any amendments to their own constitutions which would trump the ultimate authority of the US constitution.
There was no leaving the union under our constitution without the US government passing legislation, or possibly even an amendment, to that effect because the sovereignty of the federal government could not be abrogated without the federal government's permission.
The Supremacy clause also gives the Supreme Court the power to determine constitutionality of acts of the states and federal government because it can determine which federal laws and acts are proper under the authority granted to them by the constitution and which state laws are made contrary to the constitution. It's really not that complicated.
There were two choices for the South if they wished to leave the Union:
(1) they could negotiate their way out or
(2) they could fight their way out
They chose to fight, and they lost.