Why don’t you tell us of another “right” that was in danger, that should be fun. Remember or look it up the federal government was TINY in 1860 and we barely had an army.
States right’s = political power. The civil war was fought over which “powers” belonged to the Fed and which to the states.
The right to own another human being was but one of many powers the states believed had been delegated to them by the Constitution. Others being, The North subjugating the southern states to Federal “Land laws”, The national “draft” as opposed the the southern states belief that troops could only be raised thru requisisition to each of the individual states, protective tariffs and nullification, the “Tariff of Abominations, and above all the Souths belief they had the unilateral right to secede from the union.
There are plenty more.
In this context, the only right that matters is the right to be independent, which is asserted in the Declaration of Independence. If natural law gave us the right to secede from the United Kingdom, it gave the Southerners the right to secede from the Unite States.