[TChad]

It wouldn’t do any good to say that there were slave states in the Union, or that the war was about preserving the Union, not slavery.

I think that's more right than wrong, but those two do not separate neatly.

From Lincoln's Second Inaugural Address:

One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war.

[BroJoeK]

TChad quoting Lincoln:

"One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it.
These slaves constituted a peculiar and powerful interest.
All knew that this interest was somehow the cause of the war."

A well known Confederate leader agreed with Lincoln:

"...any man who pretends to believe that this is not a war for the emancipation of the blacks, and that the whole course of the Yankee government has not only been directed to the abolition of slavery, but even to a stirring up of servile insurrections, is either a fool or a liar."
--The Vidette, camp newspaper for Confederate Brig. Gen. John Hunt Morgan's cavalry brigade, November 1862.

Of course, that was before Lost Cause mythology took over Southern imaginations.