You're right about the frequent lack of realism in political arguments, Jack. Politics is generally driven by emotions, not reason. Most people defend what their emotions tell them is "right", and reason be damned...but they will concoct various forms of logic to serve their emotional bent, whatever it may be.

To a Southerner who believes that the Southern states had a right to secede from the Union (and that's a perfectly debatable proposition), the War Between the States was a war of Northern aggression...because it was an attempt on the part of the North to restore the Union by force. That involved invading and occupying the South. That involves being an aggressor, as you are the one doing the invading and occupying of someone else's home.

Therefore from a Southern point of view, it was a war of Northern aggression. It would not have occurred had not the North done 2 things:

1. attempted to maintain Union garrisons like Fort Sumter within Southern states' harbours, and...

2. invaded and occupied the South in order to restore the Union.

(The matter of slavery came into it considerably later. Lincoln made no legal move to free the slaves until about halfway through the war...when he saw that it would be politically advantageous to do so for various reasons. The North fought that war because it regarded secession as illegal, not because of the slavery issue. The South regarded secession as legal...because it was their decision! Everyone always thinks his own political decision is legal.)

Over such disagreements as what is legal and what is not and who has the right to do what, wars are fought.

There's always a powerful element of irrationality in such disputes, in my opinion. People's judgement is clouded by culture, resentment, old grudges, and self-interest.