I think what happens is that the reasons for secession and the war get conflated.
Secession occurred over slavery, over the Deep South deciding that they needed to separate from the increasingly belligerent abolitionist faction in the North. John Brown's aborted slave uprising and the widespread support he received in the North was the most blatant example of a decades long campaign of hatred against the South. Thomas Fleming wrote a good book on that subject A Disease in the Public Mind
And while Lincoln wanted slavery to end, his announced reason for going to war was to preserve the Union, something which he reiterated many times. But in the modern mind the Civil War becomes a war launched with the intended goal of ending slavery. Now this is an interesting thought, because they in effect are saying that Lincoln decided to engage in slaughter and destruction in order to further a political goal, something Obama hasn't quite yet managed to try, although there's still time. And with respect to Lincoln and Obama I've been trying to figure out if the emancipation proclamation is any different from an executive order. But I digress.
In resorting to war Lincoln was doing something more in line with Buchanan's long forgotten Utah War and Andrew Jackson's threatened invasion of South Carolina over nullification. Both of those were exercises in the national government using military force against territorial and state governments to compel compliance with already existing law, they weren't attempts at using military force as the means to change already existing law, which is what a war started to end slavery would have been.
Deliberately, and with malice aforethought.
And while Lincoln wanted slavery to end, his announced reason for going to war was to preserve the Union, something which he reiterated many times. But in the modern mind the Civil War becomes a war launched with the intended goal of ending slavery.
Again, the deliberately intended effect. It does not make one look good to posterity to announce you slapped chains on a group of people who wanted independence. Claiming that you freed people sounds much better, even if that wasn't the purpose for which you started the war.
So many participated in the oppression of the South that they have an emotional need for rationalizing what they did, and their only claim to "goodness" is making the South into monsters which they defeated because they are the "good guys".
They simply refuse to accept that they are actually "the bad guys."
Thirdly, the absolute command of Congress over the militia may be destructive of public liberty; for under the guidance of an arbitrary government, they may be made the unwilling instruments of tyranny. The militia of Pennsylvania may be marched to New England or Virginia to quell an insurrection occasioned by the most galling oppression, and aided by the standing army, they will no doubt be successful in subduing their liberty and independency. But in so doing, although the magnanimity of their minds will be extinguished, yet the meaner passions of resentment and revenge will be increased, and these in turn will be the ready and obedient instruments of despotism to enslave the others; and that with an irritated vengeance. Thus may the militia be made the instruments of crushing the last efforts of expiring liberty, of riveting the chains of despotism on their fellow-citizens, and on one another. This power can be exercised not only without violating the Constitution, but in strict conformity with it; it is calculated for this express purpose, and will doubtless be executed accordingly.
Wow. The guy was a prophet. He got a few bits wrong, but most of it he got right.
And with respect to Lincoln and Obama I've been trying to figure out if the emancipation proclamation is any different from an executive order. But I digress.
No, it is not.
Both of those were exercises in the national government using military force against territorial and state governments to compel compliance with already existing law, they weren't attempts at using military force as the means to change already existing law, which is what a war started to end slavery would have been.
All points too subtle and too far away in time to be meaningful to most modern Union apologists. The only thing they know for certain is that they want to be "the good guys". That is literally their motivation and level of intellect in these discussions.
Any point you put forth is met with a variation of "But we're the good guys!" That is literally all you can get them to say on the issue. "We're the good guys!"
They have a mental block against accepting that they weren't. With that as their starting premise, discussions usually don't get very deep or even honest. But I suspect you are well aware of that.