Love the actual history getting downvotes here... this also doesn't conflict with it being about slavery. The thing we shouldn't do is equate "about slavery" in the way the Confederate states meant it when they seceded, with "about slavery" in the sense of abolition. Lincoln did not enter the war to emancipate slaves and fight for abolition, his first inaugural address on the eve of war leaves no question, a direct quote:

"I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so."

Lincoln's primary motivation was keeping the Union together at first, and obviously that changed, because we have the Emancipation Proclamation. The moral issue of slavery was hugely important for the North's motivation and for people to fight though, many being emancipated slaves who understood the true point of fighting more than their northern white commanders, and who also faced racism from other northern soldiers yet still fought with them. The point is it wasn't some goodness of the government that defined this war to be about slavery, it was actually the slaves that did that and those that were sympathetic to this cause.

Barbara Fields is an expert on civil war history and makes the case for this view in this excellent interview: https://www.youtube.com/watch?v=_ncnTNkeoOM The question of Lincoln's motivations at the beginning of the war as Union before slavery and whether he can be excused is addressed at 55 minutes.

"it was the battle for emancipation and the people who pushed it forward... it was they who ennobled what otherwise would have been meaningless carnage into something higher. When a black solder in New Orleans said "liberty must take the day nothing shorter" he said in effect that when we count out those who have died and survey the carnage is must be for something higher than Union and free navigation of the Mississippi River"

Spotswood Rice, a former slave, writes to Kittey Diggs, 1864:

I received a letter from Cariline telling me that you say I tried to steal, to plunder, my child away from you. Not I want you to understand that Mary is my Child and she is a God-given rite of my own. And you may hold on to her as long as you can. But I want you to remember this one thing, that the longer you keep my Child from me the longer you will have to burn in hell and the quicker you’ll get there. For we are now making up about one thousand black troops to come up through, and want to come through, Glasgow. And when we come woe be to Copperhood rebels and to the Slaveholding rebels. For we don’t expect to leave them there. Root nor branch. But we think however that we (that have children in the hands of you devils), we will try you the day that we enter Glasgow. I want you to understand Kittey Diggs that where ever you and I meet we are enemies to each other. I offered once to pay you forty dollars for my own Child but I am glad now that you did not accept it. Just hold on now as long as you can and the worse it will be for you. You never in your life before I came down here did you give children anything, not anything whatever, not even a dollars worth of expenses. Now you call my children your property. Not so with me. My children is my own and I expect to get them. And when I get ready to come after Mary I will have both a power and authority to bring her away and to exact vengeance on them that holds my Child. You will then know how to talk to me. I will assure that. And you will know how to talk right too. I want you now to just hold on; to hear if you want to. If your conscience tells that’s the road, go that road and what it will bring you to Kittey Diggs. I have no fears about getting Mary out of your hands. This whole Government gives cheer to me and you cannot help yourself.

(It's not known if Spotswood had a showdown with Kittey but there are property records indicating he lived with Mary and his wife after the war.)

Edit: It's people downvoting historical letters from freed slaves and historians reading testimonies of black Union soldiers that makes me think my time on this website is just about over...