Lincoln's First Inaugural Address is a fun read for destroying the regime narrative. Lincoln is the only sitting President to endorse an amendment to the constitution to protect slavery:

I understand a proposed amendment to the Constitution--which amendment, however, I have not seen--has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service. To avoid misconstruction of what I have said, I depart from my purpose not to speak of particular amendments so far as to say that, holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable.

Every side in a war has it's own narrative. The Southern narrative is that it was about their peculiar institution of slavery, partly because the anxiety over whites being genocided as happened in Haiti was very motivating for the war effort. The Northern narrative is that it was about preserving the Union as Lincoln addresses here:

It follows from these views that no State upon its own mere motion can lawfully get out of the Union... I therefore consider that in view of the Constitution and the laws the Union is unbroken, and to the extent of my ability, I shall take care, as the Constitution itself expressly enjoins upon me, that the laws of the Union be faithfully executed in all the States... The power confided to me will be used to hold, occupy, and possess the property and places belonging to the Government and to collect the duties and imposts; but beyond what may be necessary for these objects, there will be no invasion, no using of force against or among the people anywhere.

Ironically again, progressives have taken up the Southern narrative of the war, except they are disappointed the South didn't get the Haiti experience.