The northern states, not being complete monsters or wanting to treat fellow human beings like property, refused to let anyone join their side who wanted slaves.

So obviously they are wrong because the Union already existed with the confederate states in it but let’s give this person the benefit of the doubt and say they meant after some of the slave states left (hard when they are dumb enough to think it was over slavery) as you can see from the image. You have grey states for Union states with slavery at the same time as the confederacy existed. Even if you want to ignore my explanation for why the Confederacy left the Union (not kicked out, not unable to join because the Union was moral) they are straight up lying by saying there wasn’t slavery in the Union

Again the only people that say it was about slavery are racists who want slavery to return . Everyone else is smarter than that