Those who state Lincoln was indifferent to slavery ignore his masterwork completed in the last year of his life even though the war was still on; the 13th Amendment to the Constitution banning slavery everywhere.
Those who state the War Over Secession had nothing to do with slavery ignore the entirety of US politics up to that point and the compromises made to appease the South and avoid secession.
They also ignore the fact that slave-grown cotton was the backbone of the Southern economy to the point that the great majority of their elected politicians, for decades prior to and during the war, were from the slave-dependent Plantation Aristocracy and made their policy decisions accordingly.
Lincoln was on record as opposing slavery and the expansion of that institution as a freshman member of Congress as early as 1847 when he was one of the few who would question the motives and constitutional validly (ironic I know) of our entry into the Mexican War.
This unpopular stance on principle cost him reelection, and the memory of his stance thirteen years later likely made his election as President in 1860 an exceptionally bitter pill to the South.
I think it safe to say that, without slavery, the War Over Secession would never have happened.
