We disagree on what constitutes legal secession so we won't go there, but ignoring laws that the state disagrees with? Article VI says that the "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." A state cannot nullify a law made under the Constitution and remain in the Union. If you can't live within the agreement then petition the other states to secede.
What law did the south nullify? And if a state did nullify a law, where does it say in the constitution that they will be attacked from Washington D.C. for doing so? I know you always point to Fort Sumter in S.C., but what did Virgina have to do with that? Was D.C. a bunch of southorn racists like ....