I believe the South was standing up for what they considered their own best self interest. I can't think of any other reason to risk being attacked, then having to actually fight, a major war.
There is legitimate self-interest and mercenary self-interest. The latter is not a good reason to start a war.
For Diogenes, it's always about mercenary self-interest for the North. He never examines Southern motives closely enough to see the mercenary component.
Remember, it hadn't been that long since New York, New Jersey, New Hampshire, Connecticut, Pennsylvania, Massachusetts, Rhode Island, Delaware, and Maryland had voted to enshrine slavery in the U.S. constitution.
I'm not sure what theat has to do with anything, but what "enshrine"? They accepted the status quo and gave South Carolina and Georgia the guarantees they wanted.
And actually, it had been a long time. Consider what the US was like in your grandfather's day and what it is today.