I would agree if you are arguing the constitutionality of the war, a it must be contemporary law or interpretation of existing ones of the war and not one ex post facto.
You made a declaration:
You can't declare youself not part of the union following the constitution
I asked:
Which pre-1860 law prohibits secession?
Which part - article, section, clause - of the Constitution prohibits secession?
These questions directly address your assertion.
Can you answer them?
If not, then given the 10th amendment, how does your assertion hold water?