We "subjugated unwilling populaces" when we defeated Germany and Japan. That didn't make it wrong.
The South's motives were plainly greed, but the North's motives have been deliberately disguised as some false moral crusade.
The secessionists motives have been deliberately disguised as some false libertarian crusade. And that goes on until today.
There certainly was a moral element in the US's going to war. That can't be minimized or ignored -- however much crackpot Marxism you apply.
The North's crusade spilled a very great amount of blood. The South's practice created human misery, and no doubt spilled some blood, but not on the same scale as did the North.
Who started the war, anyway? Who fired first? Who is to say that the war was all the fault of the US? And who is to say that it was worse than slavery?
Secessionists went to war -- they said, among other reasons -- because they didn't want to become slaves of Northerners. They were wrong about what was in the cards, but doesn't that prove that slavery was worse than war?
Slavery was eventually going to die of natural causes, but the remnants of the North's war on the South, and the precedents it established, have lingered much longer than slavery ever would have.
Slavery could have gone on for another 50 or 75 years. Segregation went on for another century. Northern economic domination lasted about 110 or 120 years.
That economic domination wasn't all the doing of the Yankees, but it looks like in terms of duration, though perhaps not of severity or injustice, the two were about equal.
Furthermore, throughout the South votes from Southern Unionists plus freed slaves formed local & regional majorities, reducing claims of alleged "subjugated unwilling populaces" a fraud & sham.